Help with aspire timeline 4820T

Hi guys,
I'm having trouble with my aspire timeline 4820T. It boots ok, but 3-4 minutes after startup I hear a weird noise which I'd describe as a noise of short circuit coming from inside the laptop and then it just switches off. I've opened it but I didn't manage to find anything burned(simple visual inspection). One interesting thing to note is that after cleaning the cooler and replacing the thermal paste, the machine worked for around 2 weeks and then started having the same issues. I've also removed the memory sticks and replaced them with known working ones, removed the cd drive and wireless card, the issue persists.
Has anyone else had a similar problem, or any ideas as to what it can be? I really don't want to buy a motherboard to find out it was the power key or the cpu.
Thanks!

 

Sounds to me like your thermal protection circuit is kicking in. It could be that your heat sink isn't making good contact with the CPU. Generally you can put the heat sink, minus any thermal paste in place, and then see if pushing it down gently with your thumb results in it going down lower as the support legs bend up. If this is the case gently bend them up until they are just a hair's width above their mounting posts when the heat sink comes into contact with the CPU. Then re-apply paste and screw in place.

Use a program like hwmonitor32 to keep track of temps if you can. Run it on top of a big fan or something to see if you can get it to run long enough to install / show you the temps if you can. Then look at temps after re-pasting / bending the mounting legs up a bit.