Same problem, & bought around the same time with the same model.
We went Mac all the way after leaving Christchurch, & really regret the purchases, we got 2.
My iMac does the full noise fan, with adled CPU so you can't do anything intense on it.
I used to be able to cure it with the SMC reset. But this got annoying sometimes having to do it every 20 mins.
I took it back to the shop three times before I finally just started screaming at them. They took it for a few weeks & replaced the motherboard. That cured it for a month, & then it started again, slow at first, but now it is back to pretty much as soon as I turn it on, right before Easter when I wanted to get some serious gaming in.
They blamed everything, the fact I had Windows 7 on it via boot camp, the fact I had non-apple ram I bought from OWC, the fact my apartment was not air-conditioned, then when I told them it was, they then said that air-conditioning had be know to cause issues... that was when I started screaming.
pretty over it at this point, we bought Adobe for Mac, Office 2011, cost us over $17,000 for the 2 iMacs & software. I am now looking to get the thing fixed & sell them & go back to PCs with a Mac mini to just plug into the TV for iTunes & our pics.
I Love my iPads, iPhones & iPod; but my iMac needs to be drop kicked! My wife's one had the DVD rom not reading. It was pretty humiliating taking both iMacs back to the shop within a week, & then being told we would have to leave em there for two weeks as we had not bought them from there & it was our fault for using Apple Store online.
FML!