For the misfires it may mean that one or more of the following has happened:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer

Possible Solutions;

If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.

If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

Ypu did not state any symptoms, just said acting up. What does acting up mean? If you have been feeling roughness while driving, this Mazda has been known to have sticking egr valves causing P0300 misfires. Try unplugging the egr valve and see if the problem seems to go away. The egr valve being disconnected will show a fault code for an open circuit but thats ok for diagnostic purposes. If the egr valve cannot open (because it is disconnected) it therefore cannot become stuck. That is the diagnostic strategy here. If the roughness is gone with the egr unplugged then replace the egr valve.