I just recently built a totally new-fangled system to lastly restore my previous machine (dating back to 2003).

It included the following:

AMD Phenom X4 9850 Black Edition
Gigabyte GA-790FX-DQ6
2 x 2GByte Mushkin DDR2-1066 5-5-5-15
Club3D HD4850 with Arctic Cooling cooler
Creative Audigy 2ZS
Adaptec 29160N SCSI controller
Thermaltake Toughpower 750W cable management
Maxtor Atlas 10k5 73GB
Western Digital 750GB Green Power
2 x NED DVD-RW 20x
3.5" Floppydisk

I'm using both Windows XP x64 as well as Ubuntu 8.04 AMD64 with it, both OSes newly installed.

The problem with this machine is the following:

If I pressure my graphics card to the utmost (i.e. playing LotRO or Assassin's Creed at 1920x1200 and maximum settings), the graphics card will stop working after two to ten minutes and leave me with a black screen ('No Signal, entering standby') and soon after (1 - 5 seconds) the clock on the LCD of my G15-Keyboard and any sound output will freeze. The trouble can be recreated in Ubuntu using the unique tech demo ('Tropical Island').

Memtest86 and Prime95 do work for more than half an hour constant tension test without making something stop working. Thus it seems that my graphics card does not work properly.

The graphics card I am trying to get to work right now is the second card of the precise similar model, having the precise same error. The old card just had a fan which could not be controlled and just ran at 100% all the time (which according to the support was a clear fault and they said I should bring back the card).

Now: How do I know whether or not the card is the problem or if it's an inappropriateness with the motherboard if I cannot try out any hardware in another computer (except of my PSU, I've tried both the new and an old one... same fault)? Should not the HD4850 work like a appeal on a 790FX based motherboard?