One thought did occur to me, have the cap pressure tested.
Cylinder heads been re-torqued lately? (do the free stuff first)
All else fails maybe switch to a waterless coolant, this way you could run a 0psi coolant system.
Reason I mentioned the rear ports is it's an ideal place for steam to collect with no coolant flow to wash it away.
I feel for you on this one, the cause ain't exactly jumpin out at ya.
