mac book breaks down - starts up - breaks down -----

my 2 year-old mac book has developed a strange problem:

when it is completely turned off, and I turn it on, it works problemfree for about 2 hours. then it just "breaks down" (meaning: i sounds as if the battery is empty, and the screen turns black, its off). then, mostly, it starts by itself again. breaks down - starts up - doesn`t make it to the login page - breaks down - starts up. sometimes, when i remove the electricity-cable, it manages to start on again, then the macbook runs for an hour or half an hour or so, before it breaks down again. Now dataloss yet, luckily...

Does anybody have an idea, what the problem might be? Do I have to be afraid that it is the harddisc??

I had a similar problem about a year ago. There, also, the macbook turned itself off. but it didn`t start again by itself AND when i started it, it didn`t break down again. I had that problem fixed, Applesupport had to switch some cooling-device, I don`t know exactly.

would be really nice if anybody could tell me something - before i get the macbook to reperation....

thanks