Greg: One other thing is to use a utility like WCPUID, if you're haven't already, to check what the real speed of the CPU is. As you may know, sometimes the BIOS reports incorrectly. It may be possible that the multiplier could be too high even if the BIOS reports the speed as "correct". The ASUS board I worked on would report anything at 400 MHz or faster as "400 MHz" even though the proc was running at 500. In other words you may be overclocking the chip faster than it's stable without knowing it. This is especially easy to do if the mobo remaps lower multipliers to higher ones (i.e. 2X=6X)...probably not the case again if the K6-2 works. Just a guess. I would think the newer BIOS should heve fixed this though. Unfortunately, you need to get into Windows long enough to run WCPUID.
That article at Tom's is good. I didn't realize it was the same mobo he used. I wouldn't give up just yet. If you can get 600MHz out of the proc that would be pretty sweet. Check everything one more time (sorry to keep harping on it, but especially the voltage - not sure if that mobo uses jumpers or BIOS settings).
Good luck.