It sounds like a hardware fault or incompatibility, to be honest. Try downloading the
upgrade advisor and run it while you're in Windows 98. It will tell you if Windows has poor support for your chipset or a particular piece of hardware.
Failing that, the most common cause of random hangs and freezes is a faulty RAM (memory) module. It's a wise idea to take all of your RAM modules out and temporarily use a good, known working module to see if the freezes happen.