Computer mouse, that is!

It's one of those with a wheel in the middle, so scrolling is really easy...
until today.
Normally, roll the wheel backwards, screen scrolls down, roll forwards, screen scrolls up. Simple .

Today, roll back, screen scrolls down a bit, then stops, so roll back again, screen scrolls up!
then it alternates, screen going up and down like Sludge having a dream about some old slapper.

Driving me nuts!