When you have the manifold face of the head skimmed, do you fit the inlet manifold and split collars to see if they are now too wide? Remember that removing metal off the head, reduces depth of recess for the collars. On occasions I have had to make narrower collars to compensate, especially where someone has skimmed inlet and exhaust manifold together. Always do a dry run, by fitting collars and measuring the gap between faces, then measure the gasket. You may not be compressing the gaskets enough due to the collars holding them off then once it starts to blow there is no stopping it. Also, when skimming the exhaust manifold, refit the inlet to it and check both faces line up, using a straight edge, when you nip up the nuts and bolts that hold them together.