Macbook keeps overheating

I have a white macbook here that just overheats way too quickly. I took it ll apart and changed the thermal paste and it still did the same thing. I then changed the whole fan unit which includes the heatsinks and still nothing. I turn it on and try to get to lion recovery and the farthest I can get is the apple logo before the fans go crazy and I get the please restart your computer. The heatsink is xtrememly hot to the point where I bet I could burn myself with it. In addition to new thermal paste I have tried taking out the hdd and changing the ram but it always does the same thing. I can not figure out what the porblem is.