I changed the skin - thanks for the direction giving. Following the change of skin the overlapping lines were still visible.

The problem doesn't happen anymore since i restarted the PC.
It's running since a couple of days now without any problem!
Thanks for the assistance.

(P4 3.0GHz - 1024MB - ATI Radeon X300SE 128MB - CRT 1600*1200 @ 85Hz + CRT 1280*1024 @ 85Hz)