P0300 Code

Truck runs rough, we've done everything from changing wires, and all. Checked all hoses, checked intake valves as well. My fiance is a mechanic and he is stumped, he has no more ideas left to try and fix this issue. It doesn't run rough all the time there's days it runs smooth, then there's days after it rains that it takes awhile to crank the engine up. I can rev up the RPMs and the truck just won't move, when it finally goes it****** until you get the RPMs up and hit about 50mph. Took it to a Chevy dealer and they ran tests told me to get the EGR valve and fuel system cleaned. The EGR valve was cleaned and put back, still have the issue. What can we do next? Everything I'm reading about we've done and it still runs bad.