M, Head gaskets do not dry out. You may have electrolosis causing pits in you head or the head bolts have lost there tension. The head bolts are made of tempered steel and are angle torqued. Always replace the bolts when removing the head , they are only good for 1 time only. I also like to use Dexcool antifreeze, no electrolosis problems.
Also, was the engine ever overheaded ? S.