I am having a very similar issue on my Shuttle HOT-555A motherboard (just your garden variety 430VX motherboard, nothing fancy or "out of the ordinary"), whenever I set the motherboard to use a Pentium MMX, one of the regulators would get scorching hot, so hot that this board became somehow unstable (such as random data corruption bugs when installing Windows 98, locks up systematically ⅓ of the time during a cold boot... And sometimes it would straight up reset by itself 😦). And weirdly enough if I put a regular (Non-MMX) Pentium, everything goes back to normal, with the normal Pentium, I noticed that the offending VRM only gets slightly warm, so I guess I'm onto something there.
Now I've noticed that there are three distinct regulators on my board, two by the CPU socket (these two never get hot or warm), and another one which is not too far the memory slots, this is the one that get very hot whenever I set the board to use a MMX Pentium.
And to top things off, this board has no heatsinks for the VRMs whatsoever... I don't know how should I go about that, try to unbolt the regulator, then fit a heatsink on it and bolt it back on or set up a fan blowing some air over the offending module?
This really is grinding my gears not because I paid anything for this board (got it for free, only paid 10 bucks or so for the Pmmx) but because I bought a CPU to max it out only to find that my motherboard isn't able to cope with it even though it's listed as compatible with my board.
So I'm going to follow this topic, because I'd like to see if there is anything that can be done about these toasty regulators.
Proud owner of a Shuttle HOT-555A 430VX motherboard and two wonderful retro laptops, namely a Compaq Armada 1700 [nonfunctional] and a HP Omnibook XE3-GC [fully working :p]