USB Mouse shuts off/on

USB M/S Intellipoint optical mouse.
Clean install of XP Pro w/SP1.
Athlon 2600 and Soyo board; new hard drive.

The mouse starts moving slowly; often it turns off for 2-3
seconds (with a two-tone chime when it turns off, and
again when it turns back on). PS2 port or USB, doesn't
matter. It happens during games or on the desktop. There
are even occasional lockups where re-booting is the only
cure.

I thought the mouse was going bad, so I bought a new one
(the same model) and there is no change. I installed new
drivers that came with the new optical mouse and that had
no effect. This problem only occured after I installed SP1.

Any help? Thanks in advance...