My Acer Aspire 5633 shuts down occasionally

Hi all,
I have strange problem with my laptop. It shuts down from time to time, without clear reason. I sent it to Acer repair center twice but they didn't find anything. The problem is I cannot reproduce the problem.
Example: Two days ago I have installed Linux. During the online update the laptop shuts down. I restarted it, I started the new update and again shuts down. I reproduced it couple of times. Next day I did one test. I overload the CPU and the memory - the laptop didn't stop. It was heavy overloaded but without a problem. Yesterday I have installed other version of Linux. During the online update it stopped. After the restart, I didn't do anything but the laptop stopped again after 5 minutes.
I have no idea where could be the problem, I cannot reproduce it. I don't think it is overheating. Could it be false memory or what?
Thank you very much in advance.