Idle high then low then normal got a P0300 code

I have a 2002 Chevy Malibu LS 3.1 with about 451,166 miles. This is whats happening so far the car has been acting crazy it idle high then low then normal then go crazy again at a stop the car has shut off for no apparent reason it starts right up after I put it in nuetral. The last time it did that it was the ignition module getting hot and shutting off the car I have replaced the fuel pump (was going bad). Mass air flow sensor I thought this would fix the idle it did for a few days, new plugs and wires ac delco (got a po300 code) there was a bad injector #3 (got a code for this too) I replaced it. After all that the car still runs rough I realize its not a new car nor will it run like new but i'm really trying to keep it alive. Any suggestions on what else it could be?