If you do a search on this site, or just look down the paqge, you will find a zillion of us with the same problem.

Generally, it is probably the clutch in the compressor that is worn out and not engaging. It is recommended to replace the whole compressor, not just the clutch.
Some have removed a spacer between the clutch and the compressor.

You can read what others have tried and done.

There are a/c temp sensors at the grab handles above the front doors. (Little circles with a line through them. Some have tried cleaning those.

I turn mine off at long red lights because on on accelleration it tends to blow warm. Then I can just turn it off for several minutes then on and ot willl work again.

If you haven't yet replaced the evaporator then that is usually the first A/C part to go. Will start leaking so badly that it eventualy won't work at all with a new recharge. It goes from 2 months recharge, down to a month, the two weeks, then not at all. None of it is cheap. If you are willing to remove your whole dash yourelf there are detailed instructions on a couple of sites with various links around here. www.volvo-forums.com keeps one on top of their 850 section.
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'95 854T 125,000