Hello,
Briefly, I have gone through two (used) compressors in about 3000 miles. '94 940 wagon w/ seiko compressors. Here is the AC history:

1) Leaking refrigerant -- couldn't find leak
2) Replaced all components from a parts car w/ a good AC system. It had a replaced compressor with an unknown year/mileage, only 120k on the car. (After doing more digging, it was my first compressor that was leaking.) Evacuated for 45 minutes or so, and added oil but only via compressed cans, did not drain oil from replacement compressor, though.
3) AC worked well but within a week or two it was making some screeching noises that would go away after a few seconds -- didn't sound like a belt
4) Compressor failed after about 2000 miles. It would work and then the clutch would start slipping after it warmed up (a few minutes).
5) Replaced compressor at a shop w/ a used compressor from a volov mechanic who said it was in good shape and off of his totalled personal car. Shop claimed to have replaced oil and fully evacuated before recharge.
6) 2nd compressor worked well, started making the grinding noise occasionally. After 1500 miles it is starting to slip, too. It made a horrible grinding sound right before I noticed it was starting to slip. I can now hear what sounds like bearings riding rough inside the compressor. I've stopped running the ac.

What could be causing all of these compressor failures? Is there anything I can do to salvage this compressor? What should I do before putting in another compessor?

Thanks!