We have found that NVidia graphics systems are not reliable. The ATI cards are more expensive, and not as often built on a motherboard, but MTUSUPPORT swares by them... and swares at NVidia.
If you have an ATI card, try replacing your NVidia with it and see if that fixes your problems.
If not, at least read these FAQ Center articles:
Hoster sweeping too fast on NVIDIA Chipsets
and
CDG song will not play properly from disc or hard drive