When you insert the ac adaptor into the laptop...does it feel loose , or the power led winks?
just wonder if it might be the cpu heatsink
could be that the previous owner spilled something on it...maybe had it plugged in ac outlet, had a power surge, enough to upset the laptop. could be some other things, short circuit, loose power connector..
does it have the 8600m gt,,,you know nivida had problems with it
its sure a strange problem, and a tough one to solve