Sadly its back again, tho much less annoying and I think I may have
damaged the Pentium III beyond repair..I think the timing has driufted
somewhere and teh chip is getting stick in an interrupt service routine
sometimes.

A couple of things spring to mind.
Not the mouse, as it happens with more than one. Not the optical sensors as
double clicks are being activated.
Maybe a faulty mouse port? Try a serial mouse, USB mouse - one different to
current one.
A stuck key (or 2) on the keyboard. Have seen this more than once. Alt
stuck on mine, and I caught on when pressing F4 as it tried to close the
program or tab shifted to another one.
Does it only happen when you are working on it, or does it happen when you
aren't touching it? If it happens when you aren't touching it then I'd look
at the mouse port, otherwise the stuck keys. It's always a good idea to
give the keyboard a clean anyway, but if it isn't dirt, just sticky keys
then a good sharp stabbing on the usual offenders may help. Alt -esp if
windows are closing, but also Ctrl, arrow keys, Backspace, tab, shift. Do
you have accessibilty options switched on, which allow keys to control mouse
movements and if one key is stuck may generate the movements etc. Or an
alternative KB if you have one lying about.

If the kb is really bogging a good scrub with with soapy water is all else
fails. Just don't put it up against the radiator to dry out, as a colleague
of hubby's did. Went limp and he ended up with a banana shaped KB.

PS what makes you think its getting stuck in an interrupt service routine?
Thats not easy to diagnose.
Suzanne