Engine low power?

My 03 Impala began losing power under load, I had code scan done by an ASE shop. They said the codes indicated a misfire and overheating. They said that the intake manifold gasket was leaking coolant. I changed the intake manifold gasket and the T-Stat. I check the EGR valve and found that it was apparently stuclk because I checked it by checking the plunger valve with a small screw driver. I felt a small "pop". I took the valve apart and cleaned it with WD40, then wiped the valve dry and used air to dry the excess oil. After installing the new manifold gasket, the engine did not accelerate any better. IT just seemed to fall flat when trying to accelerate above 18-20mph. I was driving it out of town on a 20 mile trip. After about 4 miles the check engine light came on again. A friend had a scan tool, when we checked for codes it came up with a p0300 code. This was after I had changed the plugs, the IACV, it was stuck and I messed it up taking it a part, so I replaced it. I've changed the plug wires and cleaned the plugs I changed the fuel filter and checked the air filter. We checked for a vacuum leak by using carb cleaner around the air plenum, etc. What should I check and in what order? I am retired with high medical cost and therefore I have to try to make the repairs myself. Thanks, George