I encountered the same problem with an AMD K6 450. To get around it, I changed the speed to 300 then reformatted and reinstalled windows and some of my other programs that wouldn't install with it running at 450. Once I was up and running, I changed to 400. However, I still can't run at 450 without getting error messages.
I've since upgraded to Win98 SE and still can't sucessfully run at 450. Other than that, it works fine.