Originally posted by paulwhannel
Actually you're dead on-- It's either a defective sensor like this, or a defective heat sensor, or perhaps your machine is actually overheating... Once the internal temperature reaches a certain point, your computer will sleep automatically to prevent damage. If it really is overheating, there's probably some other internal problem causing this (dead fan has been known to do it)... Or, you could be blocking the vents...
I'd look into heat as the biggest possibility, with faulty closure sensor being #2. Chances are very good it's one of these two things...
pnw