Apple Notebook Sleep/Shutdown Problem

Hey Guys,

I have recently encountered a problem that rather stumped me, a custommer of mine has an Apple G1 Notebook and here are his complaints:

-The Sleep/Shutdown screen randomly pops up even while machine is in use
-Once the screen is up, machine either locks up or screen goes blank

I assumed it was a RAM problem and since he only had 2 X 256 PC133 sticks I upgraded him to 2 X 512 Kingston PC133.

The machine recognized the ram just fine and booted up with no problem, but about 25 minutes into the session the screen popped up and the laptop locked up once again.

Any suggestions? I am strongly debating on a possible motherboard issue, but hoping to hear some good news!

Thanks In Advance,
Ilya :major