Power Saving mode

I've been playing this game called Dc Universe Online, it is a really fun game to play. I've been playing it for around 2 years now.
The first year i played everything was fine i didn't have any problems
then i got a new monitor and i noticed that my monitor would go completely black after about 20 minuets of game play and say Power Saving Mode.
so i tried to go back to my old monitor (which was a really old one) but the problem persisted
i still don't understand how a old monitor can work fine but, a new old would have problems???
So, i asked a few people on what i should do
Here are things i have done:
Defragged my pc
Updated my pc
changed power settings
Took out and put back in my (nvidea) graphics card

i done a little test with my VGA cable
i used my HDMI cable to connect my pc to my tv but the problem still persists so i doubt the VGA cable is the problem here.
Also here is everything i know about my pc:
OS- Windows 7
My pc is a HP Pavilion Elite HPE
i have two Cores
The Model is Intel Core i3 550 (clarkdale)
while playing the temp is around 50-75
internet surfing 30-40
the Tj.Max is 105
i've only started to experience this problem when i got my new monitor which is a LG Flatron w2243s
I don't remember the name of my old one
if you need more information i'll try to find it for you
Also please note i'm not that smart with computers.
Any advice would be helpful
(please note, this isn't a april fools joke:P)