Head Gaskets fail for many reasons. Such as....

1. Poor quality gasket
2. Head warped
3. Head cracked
4. Det
5. Incorrectly fitted/Torqued (caused by dirty/corroded threads in the block for example)

To name a few.

Personally, I would get the head into someone who can check it for the stuff I said. Then run an M10 tap down the holes and clean them out with brake-cleaner/airline, then refit the head with either a Genuine Ford or Payen head gasket and bolts. If you want to do the job a little more properly, coat both sides of the head bolt washers with Moly Grease before following the proper tightening sequence. That really is worth doing, but only if everything is clean.

Then, you'll be almost exactly where I am, with a setup that is putting up with 30psi etc. BTW, my gasket is Payen.