I need proffesinal help on this one: Alienware Laptop 5620p hardware problems?

I recently got a hold of a Alienware 5620p laptop. When i first got it the screen didn't work and you could hear the harddrive spinning but it didn't sound like it was booting correctly. Anyway, I took it apart (practically everything) then embled it back together and i tried it and surprisingly enough it worked like a charm. When playing around with it for the first time...after about 5 minutes...it froze on me and went back to the way it was. Since then ive been trying different things and ive actually been getting it to work a few times but i still haven't pinpointed the actually part that is causing the problem. I think it is possible overheating because it gets pretty warm underneath the keyboard. I will add more if someone here can help me out. It is really appreciated and thank you for your time.