Greg if your air compressor has many hours on it more than likely it's on it's way down! Yes, I agree as others have mentioned, you can replace, repair and chase each part as it fails, in the end the time and expense will most certainly have you thinking, why didn't I buy a new compressor long ago!
I have chased the same problems you seem to be experiencing. Valves of one kind or another, start capacitors, run capacitors, head gaskets and electric motors! After a few months of repairing this or that and the time I lost on other work repairing the compressor, I bought a new compressor! I purchased a better unit with two stage compressor and a bigger motor. I removed the electric motor and compressor from the old unit, then hooked the two tanks together, now I have twice the capacity! Twice the capacity is certainly nice when using the blast cabinet or grinding. I also find my air wrenches seem to have more torque. I'm satisfied and haven't had any wrenches on my current unit since I installed it. Just do routine maintenance to the unit and it will last another 7 years or longer!
Masters Degree in Perverted Idiocy.