I have been having some fits with mouse behavior that seemed to happen at the same time I upgraded to Linux Mint 12. So I erroneously blamed Mint 12. The issue remained with Linux Mint 11 etc. Some things I did in 12 and 11 would mask it but the issue would still remain. That it was now happening in a previously stable version on this hardware was a clue. Before I tested a new mouse, thinking it was hardware, I removed my KVM. For some reason the KVM was really behaving badly and introducing a lot of latency when certain things happened.
Without the KVM all of my problems have disappeared.
Don’t forget to test the KVM if you have one.