WIndows XP randomly freezes - what to do next ?

:upset:
My Windows XP freezes up randomly, sometimes without any programs even running. Have to power reset, wont ever come back on its own.

History: Pentium 4, AGP, Creative labs audigy soundblaster with firewire, AGP 3.0 ATI Radeon9800 Pro, 2 sticks Kingston DDR400 dual channel. started getting HALT/STOP screen with memory dump. One time I disconnected firewire M-audio 1814 interface with PC running, which may have zapped m-board. upgraded 1814 drivers, works ok.

troubleshooting: Put in new memory sticks to replace 2 256MB sticks, and then reformatted HD and reloaded NT and XP upgrade. Removed programs that caused errs, removed xtra PCI cards, remainaing cards have no conflicts. Now system runs ok , except random freezes, no err msg, no mention in the event log viewer. Replaced power supply with Connect-xUltra 600watter.

I can now: 1) replace motherboard with similar ( $49-100) older m-board
or
2) Upgrade entire guts with Intel Duo Core E8400, new m-boad, new memory and run a improved system. But that would be guessing its the m-board.

WHat is best action to do? :4-dontkno