QS 933 shuts down/restarts after couple of hours?

[freescotland]

My wife's QS 933 running 10.4.9, 1.5 gig ram, seems to run fine as always save for the fact in recent weeks it shuts down/restarts after a couple hours use.

I replaced the motherboard pram battery and ran hardware test, disc warrior and cannot seem to figure out the problem. My fear is that it is the power supply, which seem to be a bit pricey.

Anything I forgot to check other than power supply?

thanks