Kakarotch,
Lets start with the basics, does the motor and compressor turn relatively easy by hand? what is the current draw at startup and when it stalls? I have seen head gasket failures cause bizzare behavior, also check valve failures. make sure check valve is not leaking, it's a maintenance part like oil or an air filter. A run capacitor could also be failing, check the run capacitor, just google how to do that. Getting the basics covered will reveal your problem
Stephen