I tried Diablo, GUI miner and phoenix and phoenix works the best for me.
Though the guy in the thread I linked uses GUIminer so it shouldn't be a problem.

But there is an option in GIUminer to set the kernel it uses and the flags for the kernel - so play around with that and see the results. Most of the guys in Mining Hardware Comparison list post their flags so that is a good starting point and the thread from my previous post has them somewhere I think.