Problem with mac turning on and off upon startup...please help

[appnoobie]

I just updated my MB's OS to Version 10.5.8 and I am having problems upon startup.

My MB shuts on and off....on and off at random intervals. I read on another forum that I should start up my computer using Disk utility CD that came with my MB when I bought it, except I managed to misplace it.

What else can I do fix this problem? Please help!!

 

[fluffyx]

When it turns off and back on, does it seem to go to sleep mode and then turn back on right where it was before? Or does it restart entirely from the beginning (with the gray  logo and spinning gear)?

If it just sleeps, it may be a simple issue with the sleep switch (Apple charges less than $100 to replace this part).