When the compressor is in its 'off' state, have you tested the feed to it and got 12v meanig the compressor should be on?

You can bypass the superheat switch on the side of the compressor. It is a tight job but you can just about get your hand in and you dont need to solder or crimp anyhting, just unplug it and bypass the wire - its obvious when you do it but it is hard (tight) to pull the thing off. Dont bypass the diode though. I think it stops surges from the clutch getting back into the a/c ecu.

If the above still doesn't solve the problem then maybe the clutch coils are weak and overheating but if so, you normally have to switch the a/c off at the switch for a while for the coil to cool down before it works again - it doesn't cool down while the power is still going to it and it wont engage again.