iMac Power Failure

Just wondering if i could get some ideas on what could be happening with my iMac. I seems to loose power when I'm in the middle of using an application like iTunes or Safari or any application that has heavy CPU usage or HDD Read/Write like installing something from a CD/DVD or even on a mouse click or a key pressed on the keyboard. When it's idle it doesn't usually do this. When it does loose power, it will restart after about 7 seconds since i have reboot after a power failure checked in System Prefs. I can replicate this using any power outlet w/ or w/o a surge suppressor in my house but I can't seem to replicate it at work or at the Apple Store (annoying), suggesting it would be something in my house but i can't seem to pinpoint it. Could it be the iMac's power supply just being picky or something else?

Some Info: iMac 7,1 late 2007 (First Gen Aluminium). The power supply was already replaced in early 2008 oddly enough from the same problem but Apple was able to replicate the problem and replace it. The only upgrades done to it was the memory from 1gb to 4gb a while ago. I already put back in the original memory to see if this was it but the power issues continued.

Thanks for helping me out!
-jake