Mouse jumping

Hello!
A little while ago I started to notice something wierd with my mac.
From time to time the screen freezes (if i'm watching a movie or listening to music, that part is still going on as usual) and I can't type, move the mouse or forcequit any applications.
Before the screen freezes the first time the mouse works very well, but after that it start behaving wierd.
It sort of skips like 4 cm of the screen for every 10 cm i move it. Like if it teleport itself to some place else. This happens even if i move it slowly. I'e tried jerking around with the mouse settings and changed mouse pad, but it doesn't work. This get really annoying as i'm sure you understand. It's close to impossible to click small buttons on the screen...
My mac is an imac 10.5.8 2 Ghz intel core 2 duo.