Very odd radeon compatibility problem

I recently bought a Radeon 8500 BBA 64mb, and from the start I had problems. At first I suspected it was the card just dieing, because underclocking fixed it, but just a few minutes ago I stuck it in my main rig(because it had totaly stopped playing anything 3d on my 2nd rig) and it works fine, even at full clocks. What could be causing this problem? I suspect its an incompatibility, but why would it be getting worse? When it decides to stop working in my 2nd rig(only happens in 3d areas) the screen gets vertical randomly colored lines. Also, the mobo had worked fine with my 9100, but I epoxied a waterblock to it, so I cant put it in that pc. Any sugestions?

Bad AGP slot perhaps

Cant be, my 9100 worked fine even after the 8500 began giving me problems. Well, I guess I it could, but I hope not.

mainboard is probably diein, after all it is a gigabyte cough cough

Hmm, I wonder how gigabytes rma service is?