System keep rebooting

[novicejr]

Just built a computer and was trying to play blu ray but after anywhere from 30 seconds to few minutes, system would reboot spontaneously. When it did play, video was very smooth. CPU utilization never got above 35%.

ASUS P5B with XP
Q6600 overclocked to 3.0
3GB Crucial PC6400
Ultra X3 power supply
Quadro FX 3450 (cyberlink advisor gives yellow grade)
old Dell 2405 (non HDCP compliant) using AnyDVD
LG GGW-H20L
PowerDVD Ultra 8

I should mention that I stressed with Prime95 and CPU/RAM is rock stable.

So what do people think is the problem? I'm guessing the old non-HDCP monitor and much less likely the GPU. I'm hoping I don't have to waste the Quadro.

Eventually I plan to use this to convert HD camcorder footage to blu-ray but I would like to watch an occasional movie. Any help would be appreciated?

Thanks in advance.