As soon as you notice that only fresh air is coming in, pull over to a safe place, leave the engine running, open the hood (put it in park, set the e-brake) see if the compressor is engaged. If it isn't it could be for a variety of reasons. Haivng just experienced the same problem, I can tell you what it was on my 87 300TDT. First, due to wear, the gap on the compressor clutch was too large.
When I would back off on the throttle, the voltage drop would cause the compressor clutch to disengage. Second, there was a restriction somewhere in the high pressure side
that caused the compressor to shut off. That was solved by flushing the system. Third I had leaks, so over time the freon leaked out, no freon,no cooling. Fourth, a bad pressure switch! Basically the system was shot except for the compressor itself so instead of costing $1500 to fix,I got off with about $750 or so for new clutch, pressure switch, receiver-drier, expansion valve, seals, system flush, vacuum, and recharge. Hope this sad story helps, but now the A/C runs cold. I forgot to mention the vacuum pods in the dash were all bad--that was big bucks.