Have you verified that the clock dividers are set correctly? I don't know how you set it up when you underclocked the K6s, but if you're using a different FSB than the Pentium MMX then maybe something could have been overlooked in the setup. If PCI is overclocked it might cause that behavior. Could also be a power supply problem, or something else with voltage related jumpering. I know you said it's set to Auto, but I don't know that board and in general those super socket 7 boards tend to have a lot of jumper settings that need to be changed for different CPUs.

I don't know what the CPU support situation is on that board, but if there's any doubt about what CPUs it supports then the K6-2 is more likely to be supported than the K6-3, and should have lower power requirements from the motherboard's onboard regulator, so I'd probably start with the K6-2 for troubleshooting.
If you haven't already, strip down the configuration to be as simple as possible.

What happens if you leave the K6 CPUs set at 200/66 like the Pent MMX?
If you are able, try checking voltages with a multimeter while it's trying to boot, see if anything is low or fluctuating under load.
Any sign of bad capacitors? It could be in a marginally working state where it still works with the MMX but not with a faster K6.

what do you mean by clock dividers? The motherboard had dip switches on them. I can't find that the that the pci slots could be overclocked. I did tried a different power supply (via atx to at), same result. I have set the cpu (both k6) at the same speed as the pentium 200(actully not a mmx as I found out) same result. I tested the pentium mmx 166 with no issues. fineally the capacitors looked fine.

I think he means clock multipliers. I found some info and I'm basing these recommendations on what I've found. If you have further information about this revision of motherboard and how it differs from what's described, please post anything relevant in this thread.

You may not want to use this board with a K6-2 or newer for two reasons.
* Unless the board was updated for K6-2 or newer chips, it's unlikely that the auto voltage detection mechanism will configure the supply voltage for the CPU properly. Disabling this circuit and setting the voltage manually involves modding the board.
* As BF2 is hardwired closed, some multipliers will be unavailable to you. This may not be an issue, as 2x is present, and that's what you will use for max clock on any newer AMD SS7 chip.

For more, see below:
http://www.oocities.org/hackedmobo/ga586hx/gahow.htm

cnpr wrote:

what do you mean by clock dividers? The motherboard had dip switches on them. I can't find that the that the pci slots could be overclocked. I did tried a different power supply (via atx to at), same result. I have set the cpu (both k6) at the same speed as the pentium 200(actully not a mmx as I found out) same result. I tested the pentium mmx 166 with no issues. fineally the capacitors looked fine.

I meant the (usually configurable) ratios between FSB, memory clock, PCI, ISA, AGP (where present). I didn't notice earlier from the name that this is a 430HX board and it's options are somewhat simpler than a lot of later "super 7" boards are.
So presumably you're running at 66FSB, 33MHz PCI regardless of CPU, and based on the link that was posted it looks like it might not even be possible for PCI/ISA to end up overclocked.

Looks like it's due to the CPU voltage and current limitations gdjacobs linked, which aren't making the K6-2/3 CPUs happy and they might be overstressing the board's VRM as well.