KT7A slows with new CPU

I have recently updated my old Duron 700 chip to a new "unlocked" athlon 1Ghz, that seems capable of running up 1.3Ghz without an issue.

Please note that I have changed NOTHING else within the system during this, and that the new processor is VERY well cooled.

I have however found a rather large problem, which is that my hard drive access has gone to crap.

I've checked the Abit site for my KT7A and have updated several Bios settings (after installing the latest version of the bios, i.e. 3R). None of this seems to have helped.
I've even tried underclocking the athlon to 9x100fsb.

Just as a test, I've re-installed the old processor, and sure enough, hard drive access is back to it's normal rapid self.

A couple of potential reasons come to mind:
1. The processor has a fault in an area that addresses the hard drive.
2. The motherboard has a problem dealing with both a fast processor and fast hard drive access.

I'm really dubious about option 2, as I assume that the forums would be full of people complaining about this if it were true.

Any thoughts?