I'm having the exact same issues that you are having with the imac. I'm using a 2007 macbook pro and hl2 just started misbehaving today, while I was playing it last night and it ran fine. I haven't done anything to it recently so I don't know what si causing this.
I was able to get my version of steam and HL2 running normally again by downgrading to crossover games 7.1.1.
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