Monitor randomly falling asleep

Hey guys looking for some help here. I had this problem at the beginning of the summer and thought i fixed it but its back. The problem being my monitor going to sleep while I'm using it and the only way to get it back is to reboot. It first happend when I was playing Mass Effect 1 and 2. After a little research I narrowed it down to my graphics card overheating. When I started monitoring it the temp would regularly get into the 90's C and also hit into the 100's. One trip to best buy later and an extra gpu fan and it was better (I think I was also careful to take breaks and let it cool). Now more recently its been doing it occasionally. It has happend once or twice in windows but I recently started taking a comp sci class and wanted to install Linux just for fun so I partitioned part of my HD for ubuntu. Now I'm not sure if its because of this but while playing in Ubuntu my monitor has fallen asleep three times today alone. Once was when I was changing the background, once when I was downloading pictures, and once when I was trying to find out how to get the sidebar I had back. So I guess my first question would be is this a hardware problem like I think or is it because of the ubuntu. My money was on graphics card but I took it out and have been using the on board gpu for the past few hours and after the last indecent mentioned above I have run out of ideas and found myself here. Any recommendations would be appreciated.

Note: This is my first note so I apologize if im missing something or am in the wrong section or something like that.

Specs
Win 7 x64 OS
AMD Athlon 64 x2 6000+ 3.00Ghz processor
Biostar TA780G M2+ motherboard
4G ram
Nivida GT210 512mb graphics card (the one I took out)
500W rosewill power supply
 
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