Computer crashing randomly while playing video games.

I got my custom built computer for Christmas of 2014. My computer is relatively new compared to other computers but recently (past couple of months) I've been experiencing one a bug that makes me unable to play games for very long. What happens is that whenever I try to play a game every 30- 120 minutes it completely stops working and I have to hard restart. I have waited quite some time and pressed all of the button combos that should stop the game I'm playing but nothing happens. I have tried many thing like checking for viruses and updating my drivers and even updating optional updates for Windows. I use Windows 7 Pro. I have even tried removing my RAM sticks which I thought to be corrupt. Before doing this though my computer could only handle 10 minutes of the simplest games like Garry's Mod. After I took one RAM stick out I played one of the first games that started to give me problems (Payday 2) for about and hour and everything seem to be fine. For a while everything was and I played crash free that is however until yesterday (today is April 19th). My computer crashed once again. Today while I was playing it crashed 3 times each at a random length of play time. So I then decided after the second time of crashing to switch the RAM stick but at the same time still only having one in. My computer was a little slower but I think that's because I switched. Although maybe they're both corrupt, I don't know. So I have been given many theories of what could be wrong. One that I think might be the case is both RAM sticks are corrupt. Another one of my theories is that it's overheated, although I don't know how hot computers are supposed to get while playing games for long periods of time. Another theory is that my Windows is outdated. I don't know if this would cause my computer to crash but I really don't know at this point. The final and I think least possible theory is it keeps disconnecting to the internet which causes the computer to overload or something. So if your reading this please tell me what you think I really want to play without any problems so if you have anything please tell me. Sorry this is so long I just really want to get to the bottom of this so I want to give as much explanation as possible.