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81 320i heating problem

I've got an 81 320i with a heating problem.  The engine makes
heat (new thermostat) but does not seem to have flow to the
heater core.  

I checked the manual control valve (took it off) and it is 
working fine.  I verified that it opens fully when operating. 
I flushed the core and there are no restrictions.  I removed 
the water-pump and used a snake-tool to check for restrictions 
in the metal line running from the pump to the core on the 
drivers side of the engine.  None were found.

At less than 30 degrees (F), when the engine is running and the 
blower is turned on, the line running into the core is hot (almost 
too hot to hold for more than a 10 seconds).  The line exiting the 
core is ice cold.  If you leave the fan off, let the engine run 
and switch the fan on you momentarily get great amounts of heat 
until the core cools.

I flushed the engine with water and put new coolant in last winter.

Could the blockage be in the block itself (scale)?  Or, is there 
something wrong with the pump?  The pump impeller is in very good
condition.  Is there a metal plate on the back of the pump that
directs flow through the block (like on Ford small blocks).  
This pump has no such plate and the Haynes manual does not show
one.  My repair experience with this brand of car is limited and
I don't know the auto's history before 1995.

Thanks in advance.
Paul Lockbaum        lockbaum@domain.elided    715-726-2823