Sounds like your AGP bus is a little unstable. The KT600 dosen't have PCI/AGP locks, so your AGP bus speed will increase right along with your FSB OC. If the AGP bus can't handle the speed, lowering it from 8x to 4x will sometimes help (like in your case), though increasing the AGP voltage a little could also help keep up stability at 8x speeds.
JigPu