FWIW, most likely not the case in your situation, but for anyone who has ever had this problem and has used the Cocolino PS/2 mouse adapter in the past, make sure the cocolino.driver is not in your SS or US. I haven't used my Cocolino for a few years, since I got the Deneb, and recently while I was installing a new drive I found the Amiga mouse was going nuts like yours. Removed the cocolino.driver and all is well.
