Use this Permatex 22058 Dielectric Tune-Up Grease, 3 oz. Tube
http://www.amazon.com/.../B000AL8VD2

Dielectric grease is a non-conductive, silicone-based grease that's designed to seal out moisture and prevent corrosion on electrical connectors. It also disrupts the flow of electrical current, which makes it good for lubricating and sealing the rubber parts of electrical connectors. It's commonly used in automotive spark plug wires, recreational and utility vehicles, and electrical systems in aircraft.

I feel that if all that work was done at a shop,you should take it back and let them fix it.However,other things that could be faulty are charcoal canister full of fuel,faulty MAP, MAF sensor, canister vent valve,water in fuel ,cold start injector not opening I DON'T KNOW IF THAT ONE WAS REPLACED TOO,battery discharged if the engine rotates slowly the electrical components may not be getting proper current flow if the battery is close to to expiry date I would replace it.Check battery connections first for loosness .Also vacuum leak ,broken loose or disconnected wires in the srarter circuit ,jammed or sticking air-flow meter, auxillary air-valve sticking.Of course everything mentioned may not be on your model so it is a good idea to have a repair manual for your model so you can see what applies.Haynes or Chilton from an auto shop.

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

This should not be too hard to isolate if you use gallons of water and flood the top of the engine just a small area at a time. When the whistle goes away that means the water found the vacuum leak. This could be the throttle body gasket or the intake manifold gasket or a vacuum hose. Or other. use water as a tool to find the origin.

The 'reduced engine power' problems are typically related to the throttle body or gas pedal cable. Often, the trouble code P2138 is stored.

Possible causes;

- Faulty Accelerator Pedal Position Sensor
- Accelerator Pedal Position Sensor harness is open or shorted
- Accelerator Pedal Position Sensor circuit poor electrical connection
- Faulty Electric Throttle Control Actuator

On gm trucks/SUV (2007-2011) there are two throttle position sensors housed inside the cover on the side of the throttle body (they must "agree" with each other) and if they don't "reduced engine power " is the default (usually p2135 dtc), the issue is the clearance between the plastic cover and the sensors is to tight causing one sensor to contact it and get stuck, this is why 80% of the time this concern happens in cold weather conditions, the new designed tps/throttle body cover and an ECM reprogram will and does fix this issue.
Call GMC Dealer, it might cover under their enhanced warranty.

Start with the most basic upgrades; free-flowing exhaust system and improved intake on the engine. A performance chip / programmer can be added after these first two upgrades. Presumably, you have a brake controller, transmission cooler and a decent axle-ratio (at least the 3.42 ratio) for towing.

There are a few performance chips / programmers out there: Microtuner, Hypertech, Diablo Predator, Ls1edit, SPE proflash.
For example, the Predator can let you adjust your shifting based on TOW/HAUL mode. This may be what you are looking for in terms of a programmer.

It's worth taking a look at the P.C.V. valve too and it's filter.I assume you check the air filter.It was once suggested that fuel injector cleaner can help the o2 sensors and cleaning the fuel injectors are surely a good idea.I use fuel injector cleaner in my vehicle regularly.

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

I think you mean TANK LOCK.But I don't understand the ICON ENGINE part. The tank cover has a vent whether it's locked or not . I think , at least.

Do you have a trouble code? Maybe a CKP sensor

Try the emission control module. Had a similar experience and after replacing $800 worth of parts it turned out to be the ECM.

you either have a bad ignition distributor or a bad crank sensor. Its one or the other with a probability here of almost 100%. If you bought a code reader for $25 from amazon, you *might* be able to narrow down which one is the cause by what code is stored (or not stored at all). The crank sensor is cheap and easy to replace but I lean toward the distributor being your cause, but I don't wing it, I test components. In this case you would need an automotive style scope to check the crank sensor and a digital multimeter to check the distributor. And sometimes a certain code being present is the smoking gun.

is battery good?

Can you provide add'l details? Insufficient as written.

Have a load-test done on the battery to determine if good.Or if it is close to the 3yrs expiration or over replace it. Good battery power is essential to performing any other test.