Volvo: A/C question on Volvo v70 AND & general A/C question, volvo v70, intermittent problem

Question
Problem:  a/c unit compressor turns on and off by itself at no particular set of variables; and I have confirmed that compressor is turning when green a/c light is ON; and that it may stop turning with a/c green light still on; and I can get it to start and stop by turning A/C on and off on demand - if not having the intermittent problem.  This problem happens even thought it is still hot outside and inside the car; and at times when the car is cooled down.  The compressor may not come on when staring the car and not having run for days; or it may stop after 2 minutes and come right back on, or stay off 10 minutes; and it may stop after 30 minutes; or run for 30 minutes.  I usually do get a shutoff every 30 minutes and it so far always comes back on by itself if I wait.  
I took it to an A/C shop and left it all day, but they could not get it to turn off after having it for hours. figures!! A/C shop confirmed freon pressure.

Also unit gets really cold when it does work.

Q1. is there a relay or temperature sensor that could be bad or going bad; causing the compressor to go on and off by itself.

Q2. what makes the compressor turn off on a normal car once it gets to the temperature you want inside?

Q3. Does the entire compressor going out sound right, when the A/C gets so cold when it works (Volvo wants to sell me a new compressor)

Thanks

Answer
Marty,
Most have a temp sensor in the compressor. You can bypass the wiring to the sensor to see if that works. Compressor turns off when the evap temp gets to low, on a fully charged system. It turns off so the evap will not freeze up. There have been problems with the clutch gap on the compressors. As they wear the gap gets larger and the electronic clutch does not have the power to over come the large gap to engage the compressor. Best to replace the entire compressor.