Recently both of my systems had the GLoD. the slim was due to the board bending caused by my e3 clip that needed pennies to hold it on (raised my cpu/gpu temps by ~10-20 F)
I fixed it by removing the clip, bending the metal clamps that hold the gpu/cpu on more.. and that fixed it. My L01 that this happened to, I can't fix. tried bending the metal clamps more, tried increasing pressure more, nothing really worked. I'm guessing if tightening the clamps by bending them more doesnt work it'll need a reball.