System Instability unless Using Prime95

[Aakmr]

This may seem a little weird (it sure does to me anyways) but my computer likes to restart or lockup unless I run prime95 or some other equally intensive program. I've had this problem for a while now, and was hoping someone could shed some light on it.
System:
Athlon 2500+ (Barton)
1 gig Twinmoss DDR-333
80 gig Seagate
120 gig Western Digital
250 gig Seagate
Soltek SL-75FRN
Hyena 320W PSU
Geforce 7600 AGP
Win XP pro SP2

 

[tk11]

Your system may be switching the CPU to a lower frequency and lower voltage while not under load. Go start>settings>control panel>power options and select the "always on" power scheme. If this fails to resolve the problem try disabling any power saving options in the bios (cool & quiet).