As a minimum, I would install iSlayer IStats Nano (which is a free Dashboard widget @
http://www.islayer.com/apps/istatnano/ ) and monitor its many internal FAN Speeds. Ensure that every fan is spinning and each changes RPM during different heat values. IMO, a utility program to monitor / display internal temps and showing each of their RPMs should be included with each iMac. For example, another panel within Activity Monitor. But, that's a topic for a different thread.
As a low cost solution, purchase a small 6" "silent running" fan and place behind the iMac chassis. Something that is small, silent and has different physical speed settings. Install 2 x fans if they are low cost (and set them on low speed). Blowing air against the back of the iMac case will help cool it down. re:
http://www.techfresh.net/wp-content/uploads/2008/09/o2cool-mp3-speaker-fan.jpg
If a proactive person (like me), I would install iSlayer iStats Menu 3 (can be bought for under $20) and slightly increase the fan speeds. If wondering, I increase my iMac fan speeds to "minimum" 1,200 RPMs (instead of factory 1,000 RPMs) and the insides of my iMac are now 5C cooler. Some folks aren't comfortable with 3rd party software. IMO, a program like iSlayer Menu 3 (or something like it) should be included with Mac OSX. But, that's a topic for a different thread.
Internal temps / fan RPMs of my iMac:
http://i178.photobucket.com/albums/w251/Spike99-Pictures/New iMac system/FanSpeedJune25-2011.png
http://i178.photobucket.com/albums/w251/Spike99-Pictures/New iMac system/InternalTemps2010-08-30.png
Note: Above show fan RPMs minimum @ 1,400 RPMs. I now have then set for 1,200 RPMs - with same internal temp readings.
Long mumblings short... If your iMac keeps burning out internal parts, I would install low cost "silent" external fans behind it and slightly increase its internal fan RPMs. Especially if your ambient temps are higher than normal... To me, "low cost" proactive cooling improvements are worth it...
Hope this helps...
.