I read other posts where others used a can of air and vacuumed around external vents and the problem still persists.
Possibly you may not have had much dirt.
Having opened up my iMac as per pictures posted, I can say that vacuuming or using air around the vents on the exterior of the iMac will not do much to get rid of the deep dust that's buried within the fan and other places. That dust on the fan is hard and mud like in appearance. It took me some time to scrape and brush that dust out of the fan. Having excessive CPU temperatures, opening up the iMac also allows me to apply thermal paste.
So far my iMac has not experienced any shutdown, I'm still testing. I noticed that my iMac would shutdown within a period of two hours when running Parallels Virtual machine and using safari.
Today I'm running two Parallel virtual machines, one of these virtual machines have 2hr YouTube video playing, the other is installing Windows 8 updates. I also have YouTube Video on Safari and iTunes playing.
I'm stressing the machine more than I normally would during normal use and so far no system shutdown and temps are in a good range. I ran CineBench a few times parallel with these applications and no shutdown.
I find it interesting that so many people have this problem and logic board or power supply replacements have not solved the problem. Or in some cases users report that after having a Power Supply or Logic board replacement the iMac operates for a length of time only for the problem to resurface.
Do Apple service technicians clean the thick dirt from the machine before testing ? The dirt in the fan area will restrict the cooling, if the cooling is not sufficient then components are going to overheat and fail or become tempermental.
I initially though that this was a problem with the Power Supply or CPU. So far so good after cleaning components and applying thermal paste to CPU and GPU.