'93 Sable (3.8): A/C problem

I have a 1993 Mercury Sable GS with 3.8 engine. In August of last year with the A/C running, the car started bucking and then there was a burning smell
in the vents and I shut off the A/C and the bucking vanished and the car ran fine. I didn't intentionally turn on the A/C again until recently.
I find out by accident that I can run the defogger setting for ~30 minutes with no problems but then it will start doing the same thing: bucking and die (if at idle), though no burning smell. Also, I can run the A/C on Max A/C, full fan but the air temperature does not change. And, like the defogger, the car doesn't buck or die for the same timeframe (~30 minutes) with the A/C on. The compressor was replaced in 2002 and accumulator in 2003. Do you think my compressor is shot or is there another possibility?
Thank you for any ideas.
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