The fan could well be 'jerky' due to the tension of the belt that runs it. The fan and the compressor are temperature related. The fan works when the system tells it to. so Does the compressor. Having said that, some models run the fan on a timer. It is quite a complcated system. Providing the correct temperatures are being reached, and your electricity bill hasn't suddenly shot through the roof, I would say your system is ok. You ca always contact several engineers for a quote to see if it is worth paying for them to check it out for you.
Paul Higgins, July 2005