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					Originally Posted by  Gilles
					 
				 
				... pressure test the cooling system before any dissasembly. 
			
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^^ This. ^^
Bite the bullet and spend $150-$300 to have the cooling system pressure tested. Then you will know exactly where the problem is - you'd hate to pull the engine, tear down the top end, and find the heads to be fine. 
I'm not saying that its not the heads; I'm just saying that the more you know before you start tearing down things, the better.