So close yet so far to 166mhz FSB on K7S5A

I tried speedfan and cpufsb to change to 166mhz fsb in Windows, and I get mixed results. Most of the time it can't swtich, and crashes instantly or when I click on something with the mouse. Sometimes the switch to 166 is successful and it'll run heavy benchmarks for as long as I want. What's strange is that it doesn't like the actual switching, most of the time when I swtich back to 133 from 166 it'll also freeze. There were only a couple times I could actually switch between the two constantly without problems. Also I can switch constantly in safe mode, everytime. Some days I get a good success rate, and can swtich to 166 every boot. If I shut down the PC for a long time, or unplug the power, then it'll have problems swtiching again.

I don't think it's a voltage probem, as my voltages are the same when it works and when it doesn't. I have increased my Vcore, Vmem, Vagp, Vchipset as high as I think it's safe to, but it didn't help. Tried chaning PSU's, nothing. OS isn't the problem either, tried in XPpro and 98SE. Tried different hardware, unplugging hardware, different bios etc, no difference.

Any ideas?

My system,

WinblowsXP Pro
AMDxp 1700+ T-BredA
ECS K7S5A ver3.1
Infineon 512mb PC2700 DDR
Herc 8500LE
IBM 30GB HDD
SB Live value
430W PSU