Crash when I add my external monitor I got an Early 2011 MBP i7 2.2 8GB RAM Mavericks latest version

I worked with my laptop monitor and an external monitor. The external is connected via the Thunderbolt out with digital.

Since recently I figured out some crashes from time to time, when my monitor is connected. 

I used gfxCardStatus to check if it is the chip. Using only the laptop monitor I can use Diskret and Internal chip, so i think both are fine. However when I connect the monitor it sometimes breaks: split screen, then gray.

Today I worked two hours in a coffee shop, no problems. Moved back, worked three hours with my second monitor, then it started crashing. Now it runs for one hour with a single monitor, no problem again.

Anything I can do to make sure this is the problem? And how to fix it?
Turned out my MacBook suddenly developed a problem with temperature. I added something to the bottom of it so it has more air to breath and since then it was stable.