over the time {20mins} it freezes. when cooled down, it works again

i have this macbook, off eBay. was sold to me as water damaged, but i cannot see a single sign of water damage on it, none of the sensors has been triggered. machine works fully, when started from cold. after aprox. 20 minutes, sometimes less, it just stops reacting on anything, it freezes and have to be switched off. if turned on immediately, it does not pass even boot process.

if left for some time to cool down, it works again, but again for only at most 20 minutes.

what could it be? i have already disconnected temperature sensor, the silver compound was applied, RAM modules reseated and also swapped from known working modules...

something burned out on motherboard? failing fan? maybe i was cursed by someone, in that case, do someone know some good priest? :D

thanks in advance for any help...