Code P0300, random multiple misfires

Okay. Where to start. I started having this problem last summer. My truck would intermittently sputter and buck and misfire while driving when running the A/C or had any kind of load on it like a trailer. In the winter I had no problem with the truck at all. Now it's summer time again and now it's doing it as soon as it gets to operating temperature. It will misfire at idle and while driving engine code p0300. I hooked up my scanner tool to see if I could pinpoint the problem. Fuel pressure is 50 PSI constant. Mass air flow is a little on the high side at idle at 8.5 to 10 g/s. And at 2,500 RPMs is 20 g/s. I went down the list of probable causes. Replaced the fuel pump assembly. Replaced all the spark plugs and all the coils. I replaced the cam and crank sensors, replaced, both upstream 02 sensors, and replaced the map sensor and I'm still having random misfires on all eight cylinders even without any load on the motor like running the A/C or pulling a trailer. It will start and idle and run fine until it gets up to operating temperature at 204 degrees. Then it starts to misfire while idling and while driving. I'm stuck as to what is causing this problem and where to go from here since I replaced just about everything, I can think of that would cause this problem. So, I need help on how to proceed from here. Thanks.