i used to have the same issue, it disappeared when i reinstalled the game with other installer

I have this issue in q3 and also in defrag, but my CPMA works fine :/

yes

ok, thanks guys.

Same here, it happened to me like 1 time out of 5-6 with different mice and after installing the game many times, so I use win xp to play q3.
Also with +moveup wich was very annoying.

Prolly the standard directinput overflow issue.
Try in_mouse -1 or get a client that supports raw input.

thanks