Check the orientation of the gasket; is it sitting square to the heads? I have had to correct this problem several times before and in almost every case uneven torque and torqueing apttern has been at fault; either the gasket squiched itself out of seat or a bolt was not torqued up to spec and this area deformed when crankcase pressure built up.
A little RTV will help, but not too much!
Have a look at these first bro, and good luck!

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