3.1Ghz i5 iMac 2011 with OSX Lion GPU running very hot (80C) when running secondary monitor via thunderbolt

Question

Using 3.1Ghz i5 iMac 2011 with 6970M and OSX Lion 10.7, I'm having this temperature issue on the GPU diode where it was hovering around 80C on just normal task (browsing web). From iStat I noticed that the Graphic Module was taking full 33W even though I was just browsing.
But I did have a secondary display running at 1920x1080 via the thunderbolt port. The moment I disconnect it the Graphic Module drop to 15W and the GPU diod drop to around 60C.
So was it caused by the thunderbolt? Why does running external monitor taxing the GPU so much?

Answer 1

Try resetting the SMC and see if that helps. On occasion it takes 2-3 resets for it to "take."
SMC RESET
Shut down the computer.
Unplug the computer's power cord and all peripherals.
Press and hold the power button for 5 seconds.
Release the power button.
Attach the computers power cable.
Press the power button to turn on the computer.
PRAM RESET
Shut down the computer.
Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
Turn on the computer.
Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.