i have this laptop that suddenly shut down, things i tried:
1.Format harddisk and reinstall windows still suddenly shut downs
2.stay in BIOS still suddenly shut downs
3.remove battery still suddenly shut downs
4.reattached battery and remove the power adapter still suddenly shut downs
5.remove harddisk still suddenly shut downs even i wait in BIOS or in windows
6.change RAM still suddenly shut downs
7.check if there is thermal pasta in CPU still suddenly shut downs
8.remove CD still suddenly shut downs
9.the CPU fan does work still suddenly shut downs
Run chkdsk /r on your hard drive by going into My Computer > Right-click on C: drive > Under Tools Tab > Error Checking > Select both check boxes and click Accept. Restart your computer and let it run. Be sure your AC adapter is plugged into the laptop.
The next step is to review the Windows Event Logs during the time of the shutdown. Most the time there is an event that will give you some insight into the restart.
The heatsink? If the heatsink isn't getting hot then there isn't good contact with the cpu, hence the cpu overheating. Did you clean off the old thermal paste and reapply when you took off the heatsink before? Use hwmonitor for temps, your finger is not very good thermocouple.
The CPU temperature too hot can cause this, just turn off the option: AUTO shut when CPU temperature over 80 degree. But this can burn your CPU sometimes, you need to clean your laptop inside CPU and fan,