PC reboots automatically ,runs scandisk

hi,
I have a pc in my shop that is running a pentium IIwith W95, 32 mb of ram,6gb hdd. after it is running and sitting for a while, it can be 5 minutes or 25 minutes it reboots itself and goes into scan disk.I have removed all cards, swapped out memory, tried new power supply. I also checked cpu for spinning fan and cleaned all the dust off. I previously reformatted this drive and it worked fine. After I returned it to the customer I loaded Verizon DSL, has anyone had this problem with V dsl? This behavior is present with all windows applications closed, running in safe mode it does the same thing. I am lost.