Precautions:
- Have you disabled Cool n Quiet in the BIOS?
- Have you tried the latest nVidia chipset drivers?
- Already lowered your LDT multiplier?

LDT multiplier is also called HT multiplier in certain mobos, you might wanna check that. The default (AUTO) is 5x. With 4x, you can increase the HTT up to 250mhz. Higher than that you probably need 3x.

Hypertransport = HTT x LDT(or HT) multi <= 1000mhz