I have a baseline 06 Grand Prix that I've recently had problems with misfiring under load (higher RPMs, pulling a hill, etc.). I replaced the plugs and wires back in the spring and replaced the coil packs last week. When I connect a code reader, it gives me the P0300 code for multiple intermittent misfires.
Before I put the new coil packs on the car, it was doing it on almost every hill. After replacing them, I drove for a couple of hundred miles without an issue, but then is started back this week. My car has 256,000 miles on it, and it still has the original fuel pump. I'm wondering if enough fuel is getting to the motor when the car is really using fuel, such as under load. The next step in my mind would be to test the fuel pressure. What should the pressure be for my car, and since I've never tested the pressure on one, how would I go about checking it?
Other information that may or may not be relevant (or even true for that matter): It seems to be most likely to misfire once the engine gets warm, and it also may do it more often once the gas tank is nearing empty.
Any other ideas of what I may want to check?
04 Impala SS Indy: 
