I have an HPa6700y I

I have an HPa6700y I purchased about three months ago.  At the time of purchase I installed a high-end graphics card myself (ASUS EAH4870 DK/1G Radeon HD) and it had been running beautifully until now.  I have used it for gaming as recently as yesterday morning. Last night I installed a new game and was looking for a headphone jack somewhere on the main unit--not sure if I accidentally jostled something in the CPU or not, but seconds later my monitor (an HPw1901) went to sleep while trying to start a new game.  When I restarted the monitor it displayed the input status for VGA as ''no input signal'' and ''monitor is going to sleep.''  I tried re-booting and running a start-up diagnostic but the monitor fell asleep again about 3 minutes into the diagnostic. I swapped VGA for DVI to see if it was the cable, but had the same problem, and soon it got to where I couldn't even get a loading screen before the monitor went to sleep.  I can hear everything whirring in the CPU as it boots, it just doesn't appear to be getting through to the monitor.  I got frustrated and let it rest last night. This morning I put the VGA cable back in and turned the power strip on and off and basically re-connected all the cables to give it one last try.  I turned it on, and it booted up fine!  I checked my e-mail, surfed the web for about 10 minutes, then launched my new game.  I got through the opening credits and it went to sleep again!  What's going on here?  Please tell me it's not the video card.