Detecting Motherboard Failure

Hope there is someone reading that can advise me.

I have an Aopen motherboard AX79-400VN that appears to shutdown intermittently.

When running a memory test from an XP bootable disk a click or two is heard coming from the motherboard and the test freezes or sometimes reboots.

Tested power supply with a power supply tester.
Exchanged DVD drives (Disconnected one then the other) same failure
disconnected floppy (same failure)
removed modem (same failure)
connected another hard drive not booting harddrive(still starting with xp memory test disk)
exchanged memory chips with different chips. (two ddr400 256mb)
reseated cables

Is there something on the motherboard that could be causing this problem?
Is it possible the CPU is running too fast? (I have not changed anything)
If so how do I check for the withing range speed?

My gut is now telling me either the processor or the motherboard is failing. No dust and all fans running normal.