could be, also some motherboards have an automatic shutdown if it senses the processor is getting to hot. does it shut off, like properly, or does it just "shut down" give you the blank screen, and such. does it have a CPU fan? i know a lot of those pentium 3s were slot type and had no fan, i have one myself, but if its a socket type, it may be the fan. I had an old HP computer ive had for about 4 years, and if the motherboard senses the heat sink fan isnt plugged in or is malfunctioning it will shut the computer down automatically to avoid a reheat. a lot of motherboards will do that in fact. so check that. also swap out another PSU if you can, but part of me is saying CPU fan.