As the gasket ages, the gasket can dry out, shrink, and by the sounds of it in this case, begin to shrink away from the metal matrix that is sandwiched between them.
Older gaskes could often have such things as linseed oil in them to give them malleability, over time this dries out and or hardens.
Suggest a few heat cycles and a re torque, if that dosent help, get some permatex no3 non hardening aviation gasket, get it warm, ie crack the lid of it, put it in hot water to allow it to get nice and warm, mix well so its evenly mixed and runny, and whilst pulling vacum, apply it via a syringe to be taken up into the matrix of the gasket and fill the voids.