wierd problem

Ok heres the deal.... I have my AGP bus locked at 66MHz when I try and run my machine 175MHz/11.0x with a VCore of 1.70V its runs really well on benchmarks, etc. I found that when I play BF1942 at these settings I get a few strange artifacts I dont understand why, the AGP bus is locked. If I take it down to say 173MHz/11.0x it all goes away. It makes no sense to me, what am I missing here. It used to lock up solid when I tried to play 175MHz/11.0x with a VCore of 1.65 but since upping the voltagethats all cleared up. should I try a higher VCore setting or maybe a higher AGP voltage, its 1.5V now, I think
System specs are below in my signature.
If any one has any thoughts on this please let me know.

Thanks,

Sink