jrphoenix: I've seen you mention this problem on a number of posts, and this issue seems to be unique to you. I am curious about a few things...
1) What version of Windows are you running?
2) If XP, did you install DX9.0c with a Service Pack or by itself?
3) If you are running one of the release candidates of XPSP2, which one - RC1 or RC2?
I installed DX9.0c via Service Pack 2 RC2 and haven't had any of the issues you mentioned. I am curious if maybe XPSP2 has something to do with it.
I actually had a another thought as I was writing the first part of this post... (sorry if I'm rambling a bit - I need a second cup of coffee)
Anyway, do you have another rig you can put your GT into to try out DX9.0c? In one of the Doom3 preview articles, John Carmack mentioned that some OC'ed cards may not function reliably with Doom3 since it uses parts of the OpenGL path not previously used before. Hence, it will be using physical parts of the GPU that have been dormant. I would assume that the same is true for DX9.0c. DX9.0c may being using transistors previously unused on your GPU which may be flawed. I would think that you could narrow it down to a hardware issue by installing the card into another box. If you have the same issues then it is your hardware, if not it is obvioulsy not a hardware issue. Since the GT's haven't been around long, I would assume that you could RMA your card if it turns out to be a hardware issue.
Good luck with this. I hope you can get your rig working properly.