It's possible that one or more of your head studs is stretching.

When my '84 was given to me back in '91 it was because the engine had
given up due to overheating.  I simply put a different engine in but
during a postmortem of the old one I discovered one capnut that I could
wiggle before I started disassembly.  After removing the heads I found
that 3 of the studs had corroded and stretched with the "Wiggly" one
being the worst.

Or ... it could just be improper installation.  Most instructions I've
read only tell you to put black sealant on one side of the head gasket.
  If you follow those instructions to the letter you could easily end up
with a premature leak.

Mike