IMac :: Screen Turning On And Off At Random

I have a 24" 2007 iMac, the first aluminum ones, 4Gb of Ram and a recently upgraded HD to 1Tb (from a 320Gb failed drive). Recently, I have noticed that under some active usage, such as playing World of Warcraft or Excel, the screen will start to turn completely off for 10 seconds, and then back on for 10 seconds. It will continue to do this for about 3 hours, turning on and off every few seconds or 15 seconds. It will mostly turn completely dark but would also sometimes flicker. The curious thing, is that if I continue to keep playing, the screen would cease doing this after about 3 hours and then be completely normal and fine, but the next night.

The symptoms would repeat itself, and would consistently go away after active usage (3 hours or so). Imagine this, as if the iMac itself needs to be "warmed up" for 3 hours before normal usage. It would also appear that if I hit the machine (or tap it), this may sometimes bring a dark screen back to life for a few seconds. I plan to bring it to support, but I just want to get some ideas from you guys as to what this can possibly be.

Some of my guesses:
1) It cannot be software as physically hitting it or my dog barking would affect the rate and speed of the random turn Ons and Offs.
2) It could be a loose connector from my recent transportation of it while upgrading my HD, but then why would it become normal after 3 hours of usage?
3) Maybe something to do with RAM or video RAM? It is faulty, and thus only prolong heavy usage would "run it in" for the session? After not using it for a while, it goes back to its failed state?