You might wish to try tweaking your VCore up a notch and see if that helps.
I had the same problem (K6-III-333 from Fry's, 2.2V spec, running at 1.91V just fine until I tried CPU cooler software - every piece of software would lock up hard) until I bumped it up to 2.05V.
I see you're already running close (2.17V) to spec, so you may have less room to work with. If you've installed a pot to adjust voltage manually, try 2.2V? Maybe even 2.25V? (You're underclocking the CPU *and* you have a fan on it, so the temperature increase you'll get from the higher voltage isn't as much of a problem as it could be if you were going for 300 or higher... I wouldn't go higher than 0.05V over spec, though.)
The other option - again, 'cuz you're underclocking - skip CPU cooling altogether and drop the voltage as low as it'll go while remaining stable. You've got lots of thermal leeway here.
Finally, for debugging purposes - look around on Google for how to boot W98 into DOS. I always boot to DOS, then invoke "WIN" from the command line. While you're debugging, CPU cooling software that "always starts automatically" and locks the machine hard isn't a problem - just rename the .EXE from within DOS *before* you run Windoze. No safe mode required.