Don't think the AC shop you took it to knows much about the Volvo AC system. I'm sure others know more than me here so I'll let them chime in. When we had our '94 850, it was the evaporator, but your symptoms don't sound like ours. I might venture to guess that it may need some more gas...you could try charging it with one of the R134-A gas cans from an autoparts store or K/Walmart...you'll also need the hose with fitting that is either sold with the car as a kit or at least near the cans of refrigerant. It also sounds a bit like the clutch gap on the compressor might need adjustment, not a horribly difficult job but hard to get to. Do a search here...you'll find plenty on the how-to's as well as the proper diagnostics for what you're encountering. Others have mentioned a bad AC switch in the dash:
http://www.matthewsvolvosite.com/chat/phpBB2/viewtopic.php?t=4289&highlight=ac

Good luck!
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.