I saw the same thing happen to my P3 a few days ago in W2K.
I had just added some RAM and had all the slots full. Hangs up often with flash or graphics intensive pages with all the slots full. Fill all but one and it's rock solid. Not the RAM itself either, as all the sticks run fine as long as there is 3 or fewer of the 4 slots filled.
My system is completely different than yours, but just a though.
P3/933@1022
Via ApolloPro133 Chipset