In reply to Morty:
It coups be that he is not that good but an intermittent fault can be a real bugger to find.
I had one on Thursday with a compressor that was failing to load.
98% of the time with this type it's the load really in the controller, 1% of the time it's the load solenoid and 1% of the time it's burned oil blocking the servo.
After a few hours of messing about I need to change my percentages, it was actually caused by some swarf in the oil circuit that must have been there from when it was built 8 years ago, it has migrated to a pinch point and occasionally blocks the feed that opens the intake.
Because it is an intermittent fault though had it worked once I had changed the controller I would have been likely to class it as fixed.