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I was under the impression that al imacs with Tiger were freezing as well. I'd be pretty suprised to find a software fix in Leopard fixes the issue. This sounds like a firmware issue (cooling a potentially overclocked part somehow). Or, it's a physical heat-related issue that will only be fixed by a new revision to the case to provide better venting. You'd think the aluminum case would act a bit as a heat sink.
 
As it shows there is a new Video Driver 1.0 that also just came out that says the freezing issue is fixed.
 
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