Volvo Repair: Air conditioning failure on S60, tons of money, freon

Question
First of all thanks for all your previous help, you have saved me tons of money.
My question is, I have a 2001 S60 with 100k on the odometer. I runs real good, but when running the AC after about 20 minutes, it starts blowing hot. It does blow cold when running however. We have to turn it off and back on to make it blow cold.
I would appreciate some diagnostic direction here. I am thinking an electrical issue, but could this be a lack of freon issue?
You help is appreciated.
Steve

Answer
Steve, it may be low on refrigerant but I would do this test first. You must be careful doing this.  When the A/C stops blowing cold, go to the compressor and with a prybar, tap on the clutch, if it engages, then the clutch needs to be reshimmed. I would have the pressures checked as well.  Roger