[67FEfastback]

I've been dropping in on this thread from time to time.
I'd say to keep methodically going through everything.

Different car, but similar problems. Fortunately I had a pressure tester to help as well.
FE 1st boiled over. (new rad-high flow 'stat)
Added an expainsion tank
pressure tested the system
fan clutch had failed - replaced that
pulled the 'stat it had stopped opening (brand new Mr gasket)
tested the rad cap = NFG not holding pressure.

Put it all together
No more boil overs
Still hot at traffic lights, heats up when spirted on the freeway
Added twin elect. pushers
Better but it's still winter / spring.
Have a new BB aluminum raditor to go into the car, in a couple weeks.