I had the same thing happening to my Early 08 15" Mac Book Pro.
It required a new motherboard to fix it.
I had a flat rate depo repair for 350, not bad I thought for a motherboard replacement and 90 days warrantee.
Here are the details of how the flat rate repairs work.
My extended Apple Care was out by one year and the machine was old enough that they no longer repaired them at the Apple Store.
But -- it was still new enough to qualify for a depo repair at a flat rate.
The rate they will quote you will vary from 300 to 7 or 750 for a laptop if I remember correctly and what the rate will be depends on the condition of the machine.
I was given a 350 rate since the machine had only scratches on the case, no dents or visible signs of dropping, and a good display with no cracks or bad scratches (your milage may vary).
The depo will test everything and repair or replace any and all parts for the one price -- even if they have to replace the bottom case, top case, mother board, keyboard, and lid!
They will quote 7 to 10 days but mine took only two.
You get a 90 day guarantee on the machine.