With no help from Apple, Mac Fans have been trying to piece together what could be causing such a kerfuffle. Initially the thought was that it was probably some cheap Chinese component which was failing or overheating.
However when Windows 7 appeared on the scene and users started installing it using Boot Camp, they discovered, much to their horror, that the Microsoft product was driving their iMacs better.
The conclusion can only be that it is a driver problem within Snow Leopard which is not controlling the big iMacs' graphics hardware properly.
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Well thats quite hardcore!! Hopefully this can be fixed with an update.