Re: Experienced User with Bizarre Problem
Definitely agree with the cardboard rebuild. You won't hurt anything short term, and it eliminates any grounding issues, or issues with excessive pressure on the board when it's not standing on those standoffs. I usually use the box that the original motherboard shipped in to build the system on top of, that way the board fits perfectly and I can still install things like the Video card and any add in cards.
Another possibility is that you may want to set your "init display first" option to the slot where the card is installed, instead of "onboard" or "PCI" (depending on board) - I haven't seen this really make a difference, but some boards can be picky.
I'd also reset the event log, and set the BIOS to "clear configuration data" (it's a menu option usually listed on the first two sub menus of most pre-UEFI BIOS screens) - sometimes the BIOS caches config data that is bad, and if you leave it set to "clear" then the BIOS fetches new config data every time it POSTs.
The other thing to consider is power. Your 6800gt sips power compared to the 6850, and while the 6850 isn't a heavy user of power, it does use more than the 6800 will. What kind of power supply do you have feeding the system?
Also, does the system POST at all with the Radeon when the problem starts happening? I know you said it doesn't, but I just want to clarify if the system hangs up entirely, or if it's POSTing in the background, just without video.