Did you use new head bolts?
It could be that your old bolts have been tightened to their elastic limit. New bolts of the right grade, when tightened to the proper torque act as springs, and will keep things tight. If the bolt has been over tightened, it can reach the elastic limit of the steel, and no longer acts as a spring, even though it may seem to be tight. When the engine gets hot, and the bolt expands, you get a leak.
So it could be your bolts, not the gasket. Just a thought.
Kurt
