Now I've re-read your problem ... forget what I said about the heater control valve not closing tight. I thought the compressor was still coming on.

So when the interior/weather get hot, does the compressor click on for a few seconds and then click off (typical when low on freon), or does it not click on at all (like it's not getting any electrical current from the switch). If it's the latter, check the power wire for voltage. If no voltage, then at least you know you need to trace back through the wiring, switch, etc. for a point of electrical failure. You can also run a jumper wire straight from the battery to the compressor to see if that will get the compressor clutch to lock-in and genetrate cold air.

Last year, our 740 was acting kind of like this. It would work fine in the morning or evening, but during the heat of the day the compressor would "short cycle" and not not run long enough to generate anything better than "sort of cool" air. It was baffling to me, because I thought that if the system was low on freon, it would act up regardless of the outside temperature. Someone could probably explain to me about things like the specific gravity of refrigerant at different temperatures, etc, etc.

Has anyone actually hooked up test guages to confirm proper freon charging?

You might want to look for a shop that specializes in JUST A/C WORK - for ANY make(s) of autos. For a lot of mechanical problems, you want to find someone that really knows Volvos in and out. But for A/C problems, you might want to find someone that specializes in A/C only.

Good luck.