'88 Voyager heating problem (no simple fix)

I've got a '88 voyager, 3.0L v6.  I can't seem to get any heat out of it.
The gauge only climbs just above the "C" on the gauge, and it blows barely
luke warm air, and that's after 15 minutes idling!  if I drive it falls like
a rock to below the "C".  Basically, not only is there no heat  in the
cabin, there is no heat in the engine either.  I can put my hand on the
upper and lower hose and tell it's not up to temperature... even the plenum
seems cooler than it should be.

I know what you are thinking... thermostat... I'm on my 3rd one! I've
replaced the water pump, thermostat (3 times), all cooling hoses and lines,
radiator and heater core have been flushed and flow excellent.  I'm not
loosing any coolant, it's not in my oil pan, or on the ground, and it's new
fluid (mixed 50/50).

Any ideas... I was thinking air locked.. but I've tried everything to get
any trapped air out... and none came!  Hoping for a wiser person than I.
Thanks.

Jason C.

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Jason M. Cluett
[EMAIL PROTECTED]