I think that the ignition switch circuit has to be tested , but the diagnostic will only be successful if done while these things do not work. The ignition switch could be the cause or there is a problem with the power distribution center under the hood. You will need a sharp guy for this. One clue to isolating a cause is to see what main fuses are missing battery voltage when the key is switched on, but again, this has to be done when failed. If that scenario cannot be replicated you may have to just replace the ignition switch and see what result is.

First step - have the check engine light or stored fault codes scanned - for free - at any local auto parts store; Advance Auto, Auto Zone, NAPA. Then, post a reply as 'add answer' for more assistance.

Possible Causes of Engine Hesitation or Stumble:

Dirty fuel injectors (cleaning the injectors often fixes this)
Dirty air filter (change filter)
Dirty / clogged fuel filter (change filter)
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires

First step - have the check engine light or stored fault codes scanned - for free - at any local auto parts store; Advance Auto, Auto Zone, NAPA. Then, post a reply as 'add answer' for more assistance.

Possible Causes of Engine Hesitation or Stumble:

Dirty fuel injectors (cleaning the injectors often fixes this)
Dirty air filter (change filter)
Dirty / clogged fuel filter (change filter)
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires

The rattle or jerk with the a/c engaged (system on) may be related to these possibilities; faulty / worn a/c compressor drive belt tensioner, faulty / worn serpentine drive belt idler pulley, faulty / worn a/c compressor, low a/c system refrigerant charge.

Check the many on-line auto parts sources, such as; www.autopartswarehouse or www.rockauto.com for the replacement parts. For example, they (RockAuto) list the belt tensioner from around $45.00 and the serpentine (accessory) drive belt for the a/c from around $9.00 for your vehicle.

The sound is at idle speed? Does the sound change - go away at other engine speeds? Have you raised the hood and observed for yourself - with the engine 'on' in Park or Neutral - and with the a/c system operating - to see where the noise is coming from?

If not, then try this as a first step - and then report the results - by posting your reply.

A code P0440 could mean one or more of the following has happened: The gas cap is not installed or working properly The purge solenoid has failed The canister is plugged and not working properly

Here are some things to try:
Remove and reinstall the gas cap, clear the code, and drive for a day and see if the codes come back. Inspect the EVAP system for cuts/holes in tubes/hoses
Inspect for damaged or disconnected hoses around the Evap purge solenoid
Check and/or replace the sensor Check and/or replace the purge valve
Have a professional use a smoke machine to detect leaks

Unless you have hooked up a cooling system pressure tester when the engine is overnight cold and pressurized the system and then checked very carefully around all places where coolant could leak from, then you cannot know for sure there there is no external leak. One of the most common places for a leak is the metal pipes that run down the pass side of the floor under the vehicle that supplies coolant to the rear heater. Pipe usually rusts through porous and leaks just behind the pass front tire under the vehicle. If you are no leaking it then you are burning it, but you would notice a sweet smell and maybe some white smoke from exhaust at least sometimes. And lastly i will assume that there is no coolant leaking onto the pass side front floor area from a leaking heater core.

Have to scan the system first...

It could be an ignition problem, vacuum leak, fuel delivery problem, sensor, etc. The first step in troubleshooting is to scan the computer system for trouble codes.

P0457 = Evaporative Emission Control System Leak Detected

Most likely related to a leak at the fuel pump nozzle. Here's how to fix it. First, you need to have an empty or near empty gas tank. Either drain the gas out or drive around until you are on empty. You don't really need to jack the car up, that way the fuel tank can rest on the ground when you loosen it.

The first thing to do is remove two bolts that hold the back end of the muffler and exhaust pipe up. let it lower down and rest on the ground.
Next you need to remove the screws around where you fill up the gas tank to loosen the pipe.
Next you need to disconnect the fuel tank itself. There are two long metal strips that hold the tank to the car, one closer to the driver's side and one closer to the passenger side. Remove the bolts and bars and let the tank down to rest on the ground. It might help to have a jack to support the tank while you are removing the screws.
The top of the fuel pump sticks up out of the top of the tank on the passenger side. You will see several nozzles and hoses connected to the top. The problem one points up towards the front of the car. The plastic nozzle itself has probably cracked.

Here is where you might have to get a little creative. Somehow, you need to reattach the nozzle and hose to the fuel pump in such a way that it will not break again. The inherent problem is that the hose is not long enough and it pulls on the nozzle which over time leads to it breaking. You may be able to attach a longer section of hose to prevent future breakage to the plastic fuel nozzle.

Unless the pump nozzle is unrepairable - then you'll have no choice but to replace the fuel pump as an assembly.
Try the many on-line auto parts sources, such as; www.autopartswarehouse.com, www.partsgeek.com and www.rockauto.com for the replacement parts. For example, they (RockAuto) list the fuel pump from around $115.00 for your vehicle.

Probably needs a new carburetor.