I had the same problem with my S70 since new. It would only happen when the outside temperature was in the 90s and I sat in a parking lot waiting. Over the years it happen more often and at lower ambient temperatures. Don't waste your money on sensors until you determine if electrical power is getting to your compressor clutch. If all your sensors are working properly your system will be sending power to the compressor clutch to ingage. If it is getting power the problem is an internal thermoswitch that is sensing an overheat condition. When the compressor cools down the thermoswitch closes the circuit and the a/c works again for several minutes until it heats up again. I have read that some Volvo techs have found a way to short out the sensor to make the A/C work. I was unable to find one so I had to have the compressor replaced. Expect to pay around $800.

follow the recharge line under the hood on the passenger side back towards teh firewall, you will see a electrical harness that plugs into sensor in the line. this is the low pressure switch, the next time your AC compressor does not turn on, unhook this plug and jump the circuit in the harness with a piece of wire. if this turns the compressor on, then you either need freon or a new pressure switch. the high pressure switch from what i understand is incorporated into the compressor and can not be checked. does the compressor make noise or is it nice and quite when it is turning? if noisy then the compressor may be going out.

was the system properly evacuated with a vacume pump when you had it recharged? it could have air or moisture in the system and this will cause poor AC performance.


Dan
FAA Licensed Aircraft Mechanic
97 850 turbo GLT

Here's a link that should answer most of your questions:

http://au.geocities.com/ozbrick850/aircon-keithP.html

Even if it doesn't, it's good Volvo geek reading.

Luke

Outside air temp.,High pressure switch,Low pressure switch,Electrical continuity.
Check out "airconditioning" search.