The two upper-end P3 Gateway systems that I've worked on in the past do not have fans on the CPU heatsink, which amazed the heck out of me. They had plastic shrouds coming off of the power supply in an attempt to suck the air off of the CPU. Without a CPU fan it tends to make them more sensitive to heat, so on both I just bought a $10 heat sink and fan combo and strapped it on, with good results.

So yeah, heat could be a problem too.
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