Ian,
Sounds a bit familiar I'm afraid. If you do a search on my user name you'll find a few posts about aircon. The compressor was broken when I got the car so I had a second hand unit fitted. The guy who fitted it commented on the wobble and asked if the donor car had been in a crash but it seemed to work fine.
The system was gassed up by ACS (I think - label inside engine bay if its still there) using the r12 drop in replacement gas. They gave it a clean bill of health and all was well for a while.
After a while I started getting a bit of a squeal but didn't have time to look into it so I removed the fuse (rather than cutting the belt).
Last summer I went back to ACS. System was confirmed leak free again but was completely empty. Once again, after the re-gassing all was well.
I wonder if the system is loosing gas or something. It certainly doesn't seem to last long. I wonder if the system has the correct amount of oil/lubricant in there since the compressor was fitted.
David