Acer Aspire laptop random shutdown

I have this acer aspire for almost 2 years and recently having a random shutdowns. I already scanned my laptop from viruses and malware I've used avg, avast, spybot, malwarebytes but I found nothing. also I am having this weird issue when I am playing this game heroes of newrth my laptop doesn't shutdown it is very weird, also my laptop doesn't shutdown when I let the monitor off by doing nothing. please help me I already format this laptop but still having this problem.
 
I would say that is is over heating, but when you play games it doesn't shut off. Remove the battery and run it only on AC power. See if the shutting down stops
 
I always use my laptop without a battery, if it is over heating why is it suddenly turn off when I first time turn it on and why is it when I playing that certain game my laptop won't shutdown. thanks for replying :)
 
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