My A/C was working intermittently so I replaced the compressor, expansion valve and the pressure switch. The compressor stopped working before I charged the 2nd can of R134.
OK, what do you mean 'stopped working'? The clutch disengaged?
What did you do to the system after replacing those components, did you run a vacuum on it? Did you replace the receiver/drier?
What are the high side and low side pressures when the compressor is shut off? If the low side is too low, it won't turn the compressor on. If the high side is too high, it won't turn the compressor on.