Games Hanging Weirdly

Hi guys

My son has a new/rebuilt computer but is having trouble with it. He can be playing a game, such as Minecraft, for about 3 minutes and then it just switches back to the main menu all of a sudden. He can play for a few more minutes and it does it again. It is doing this on other games also.

He has just told me it also does this when he's browsing the internet using Firefox. The page just hangs and it's like the cursor disappears and he has to reclick on the page to get it going again.

The computer wasn't doing this a couple of days ago so I have no idea what has caused it, but it's to the point he can't really use it since it hangs every few minutes.

Any ideas please?

 

It almost sounds as if something is popping up on top of the apps. Like a firewall popup or a bug or similar.
Have you run cookie, Malware and Spyware cleaners?
Are all drivers correct and up to date?