Well, the answer is yes, it does have to do with the PCI bus. USB being a serial communications channel, it requires on the consistancy of Clock speeds. I lost mine when I got above 140 on this K7S5A.
As to suggestions on what you can do, maybe your board supports bigger dividers for the PCI so it will run at a lower clock. This may work, it may not. I don't know. Also, some clock chips give you better dividers when you reach certain FSB levels, for example, if my system would boot at 166mhz FSB, I would go back down to a 33mhz PCI clock, but it won't, and I'm not doing any more mods on this board, I nearly killed it by taking chunks out of the chipset chip with a kitchen knife.
There probably isn't much you can do unless you can change the dividers, or the board supports 166fsb and the chip will run at that speed. (if its a TBred, you shouldn't have much trouble so long as you up the voltage and the RAM can take it)
Linear Mode
