When my PS/2 mouse is connected via the motherboard's PS/2 connector the pointer goes crazy at times racing around the screen and randomly opening/closing windows. This can happen several times per day. This behavior is accompanied by the following in /var/log/messages:

psmouse.c: bad data from KBC - bad parity
psmouse.c: Mouse at isa0060/serio1/input0 lost synchronization, throwing 2 bytes away.

I have ruled out that this is a hardware problem. How? There are no such problems under any MS Windows variant on the same platform. More importantly when I connect the same mouse to the PC via a PS2<->USB adapter there are no problems. If the mouse HW is the culprit then one should see some evidence to that effect in either or both MS Windows and the USB configuration. The evidence suggests that psmouse.c or related code is at fault. It seems mouse signals are not handled by the same code when connected via USB.