borrrden, you said you added a monitor but you didnt add another video card, right? Mich says that you did, so maybe a clarification on that?
If you havent added a video card, which i dont think you have, then its a matter of overheating or a power supply that is starting to go bad. Power supplies sometimes just die but other times they act up here and there and cause problems but they still appear to be working ok.
Just for a test, take one side off your computer and place a house fan blowing right on the cpu\heatsink\fan setup and see if that solves the problem. If it fixes the problem then you either have to clean your heatsink\fan unit real well or get a better one. They arent expensive.
It might be a good idea to take out the cpu\heatsink\fan, clean it all up well and re-seat with fresh thermal paste, then try your system with and without the house fan, see if there is any difference.
If the fan doesnt fix the problem then perhaps it might be the power supply. If you can borrow one from a friend that is known to be good and try it out that might help you find the source of the problem.
After heatsink\fan and power supply, if you still havent found the problem, then maybe have a look at the ram.
You can download memtest from google for free and run that prog to test your ram as ram can go bad. Test only one stick at a time.
Good advice + great people = tek-tips