You've probably got a diagnosis by now - just read your message today.

I had something similar.  Turned out to be a valve seat had come loose. The
two cyl. with 70psi are on the same head I assume.

I'd like to tell you my story had a happy/inexpensive ending. Not so. Got
home from Pennsylvania to Ottawa (9hrs) where the similar symptoms started,
but the damage had been done. A burnt piston.  Ended up being cheapest/best
to install a factory rebuilt engine from VW Canada that so  far is working
OK.

I'd be interested in hearing the final diagnosis.

Alan Pickersgill
Ottawa, Ontario
Canada
'85 Transporter (modified as Westfalia)