I tried resetting the PRAM and the NVRAM. At first it seemed to work well, my computer was fine for a good 2 hours, but as soon as I got into more graphics intensive stuff, it crashed again. So, while the resetting the PRAM and NVRAM did seem to extend the time my computer was function using the external monitor, it didn't fix the problem.
Also, I'm not sure how related this is, but I figured it would be worth posting. Last night while using my Macbook Pro's track pad to pinch-to-zoom while in iPhoto, my computer crashed; pretty much exactly like it does when I am running both the laptop screen with an external monitor. It has done this once before, so out of curiosity I started up a separate photo program (Adobe Photoshop CS4) that I knew supported the pinch to zoom trackpad gesture to see if it would crash the computer. Sure, enough, it did it almost immediately. The weird thing, is that I have had practically no issues while working in photo programs (mainly Photoshop CS4), zooming in and out with an external keyboard and mouse and I do some pretty heavy editing in photoshop at work. Does any body else with these external monitor issues have the same thing going on?