Have you attempted to change the agp to 4x to see if maybe that is the problem?
It looks to me like it is a config problem, although I do NOT have the experience with that mobo.
I know you bought it for 8x, but it doesn't hurt to check if that is the problem....... plus 8X is far overrated at this time.
Is that a fresh install with no N drivers in it?
Sorry if you mentioned that it is, I checked a couple posts back and didn't see it mentioned.