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About two weeks ago, I had a sudden drop in gaming performance on my 27" iMac with ATI Radeon HD 5750 graphics. While before I could play games without any problems at high graphical settings, suddenly playing Diablo3 for example became unbearable with around 10FPS at the lowest possible settings. Oddly enough I simultaneously started having similar problems when playing games on my Windows Bootcamp installation on the same machine.

I am not noticing any other problems besides those when gaming. Rebooting the machine does not help at all.

What could be the problem and how can I solve this?

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I had this and fixed it!

Basically something was overheating. It would freeze as it hit the overheat temperature and then resolve once the temperature dropped about 5 degrees celsius.

So I downloaded some random fan speed app (link below), cranked it up to full speed and haven't had a problem since!

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That's a good app. I always set the fan speed to 6000 rpm before I play any games. – daviewales May 19 '13 at 3:41
up vote 1 down vote accepted

A thread on the Diablo3 forums suggested to try and reset the SMC first. Somewhat to my surprise, this immediately solved all my performance issues both on Mac OS X, as well while gaming on Windows.

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You should accept your answer... =P – daviewales May 19 '13 at 3:42

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