Check and make sure that your rear heater core isn't leaking.  I had the same
symptoms as you and first checked the heater core-bingo! It was leaking big time.  I
removed it and plugged the hoses, bled the system via the jack up the front, have
wife keep it running at 3000 rpm, and run back and forth between the radiator valve
and filling the expansion tank in the back, technique.   If it is something more serious
I have a spare german full 2.1L gasket set that I'll sell you for $25.

-Ryan London
'90 Syncro Westy
Chicago