Well, as I stated, I already tried a couple of drivers, even older ones as recommended in other threads.
My configuration:
Athlon XP 2800+
Geforce 5600 (using the X-Treme G Drivers v81.97)
1024 MB RAM
Asrock K7VT4A Pro Bios Rev. 1.50
Benq FP71E+ DVI TFT @ 1280x1024 @ 60 Hertz
OFP worked flawlessly with my old CRT till a few days before. I just changed the display. Maybe the problem is related to the resolution and the refresh rate. I used to play at 1024x768 at 85 Hz. Except of the resolution I didn't change my drivers setup - except that I switched off manual overclocking of my video card. But this didn't help either...