my early 2011 MBP freezes when I uncheck "Automatic graphics switching"

I have the "early 2011" MacBook Pro 15" model. Starting about a month ago, it started randomly freezing, often leaving weird graphics on the screen as if the video card had a problem. I took it to the local Apple Store and they diagnosed that the video card was bad. A few hundred dollars later, I had a new main logic board in the MBP. I took it home and the problems continued. So I took it back. The Genius did more diagnostics and eventually replaced the main logic board again. I just brought it home today.

Now I'm STILL having problems, but I've isolated the problems way down thanks to a lot of trial and error and rebooting. I've learned two things.

First, the computer instantly freezes as soon as I try to view any web page that uses Microsoft Silverlight video. I first discovered this when I noticed I couldn't play any Netflix streaming videos. But I can also reproduce the problem by just navigating to Microsoft's own Silverlight test page. So I completely uninstalled Silverlight, thinking that would solve the problem.

Second, that didn't solve the problem. The computer also freezes if I run video editing software. But I found an even simpler way to reproduce the problem. If I simply go to System Preferences > Energy Saver and uncheck the "Automatic graphics switching" checkbox, the computer instantly freezes, and I have to power it off and restart it.

I found this YouTube video where someone else had this same issue: http://www.youtube.com/watch?v=EpQCQr6IAJM

So my basic question is, "What should I do?" I've already had the notebook into the shop twice to fix the problem and the problem isn't fixed. It's hard to believe that they replaced my original bad main logic board with another bad main logic board, and then replaced that with a third bad main logic board. But it's also hard to believe this is a software problem, because in the course of troubleshooting this mess, I erased my hard drive and re-installed Mac OS X from scratch.

Any advice on how to proceed? I'm at wit's end since I'm a freelance videographer and haven't been able to actually edit anything for a month now.