a very strange graphic performance phenomenon

i recently encounter a very strange graphic performance of a v3.0.1 tbs, running on a 750mhz p3, nvidia geforce 2, direct3d renderer, xp pro, an older wacom ud0608.

during my work a drawing view is looking like this:


but some time then, after a few hours of work, a strange thing happens and the same drawing window starts looking like this:


a restart of the tool doesn’t help, only after a reboot of the computer things look in a proper way again.
it happened to me a few times since the last week, after using this version of the software without problems for 4 months (since downloading of the last revision).

does anybody have a clue?
thanx in advance.

as for today, the preference change from ‘direct3d’ to ‘opengl’ resulted in repairing of the tool performance after a mere restart (no more reboot necessary), and still looks ok.

let’s see if it was the reason of this malfunction.
what i don’t understand is the fact that the whole started only recently after a long period of proper working.