Further to my previous thread I have just noticed that the overheating problem gets a lot worse when Safari extensions are switched on. I have just put FastestTube 18.104.22.168 in there for downloading video from Youtube and it appears that this zooms up the temperature even when it is not being used. Puzzles me so can anyone explain please ?
I have tried the Apple support forum which was no help and did see the thread referred to and have tried all the solutions suggested. The iMac appears to run only slightly warm to the touch when the hand is passed over the long slot along the back and smcFancontrol tells me the problem is worse when the temperature gets to 29C. It rarely goes above 33C. The problem is getting worse and I have stopped watching HD completely and never full screen. The iMac works OK in all other circumstances and only overheats when watching Youtube videos so I suspect graphics card problems. I have ditched Fastesttube and now use Savefrom.net helper 2.33 which seems better.
I know sod all about Macs, But could it have fluff/dust/debris in the cooling slots? maybe the gentle application of a vac to the slots might work, Hose extension of course not with the actual hoover! or very very carefully with an aerosol air blower then the vac. NOTE, if you bugger it up do not blame me only a suggestion!
Thanks for the suggestions. I have vacuumed with a narrow nozzle at all available apertures and gently shaken the iMac to dislodge any blocking debris, I tried at the long slot along the top rear, the hard drive slot on the right side and the bottom slots next to the RAM.
iMacs do indeed have fans and I have installed a free app called smcFancontrol which raises the base level of fan speed. I have it currently set to 2500rpm which is a good balance between cooling and noise and much higher than the Apple setting.
I keep everything except apps on external drives and I think I am heading towards reinstalling Snow Leopard.