Brilliant!! I've de-activated the hardware acceleration and the problem is gone.
Thanks a bunch!
Since i don't know what i've just done could you try and explane it to me? Easy on the technical lingo please, cause i'm absolutely no expert...
Thanks again! Bob.

Hi guys, i'm new here. Must honestly say i registered just to share my problem, hoping someone knows what's up. The only guy i know that's sort of an expert doesn't...
Anyway, the problem is this; sometimes, after activating one of the draw-functions or the measuring tool, my mouse starts to leave a trail of colored pixels. That's really the only way i can put it but i've added a screenshot to make this more clear.
This problem only occurs on 1 mac, 17" iMac 1,83 Intel, various other models (Powerbooks, 20" Intels) do not show this problem. Not even when working on the same file. I've recently uninstalled sketchup completely (and emptied the bin) and downloaded the latest free version but allas...
When the problem occurs, as soon as i scroll/zoom in or out the pixels vanish. On moving the mouse the problem reappears.
Anyone?

Thanks! Bob.