It's probably the clutch gap thing. On Volvospeed some people had success cutting the sensor line but not 100% success and that sensor is there to protect from damage. This is about the 850 but the same applies to other models:

http://volvospeed.com/vs_forum/index.php/topic/80181-850-ac-clutch-gap-fix/

My 98 S70 had the same problem - seemed to happen more when the car was hot (heat waves, stuck in traffic, just when you need it the most!)

It seems to me this is a better fix because it is the root of the problem not a bypass. I tried it first with some washers cut a section out, did not use adhesive. My AC compressor was on all the time (even when turned off) because the washers were too thick. Luckily they came out after a few weeks anyway. So I re-did it with a cut up credit card, superglued in place this time. Bread ties did not work for me they kept breaking.