BMW Repair: No heat when moving., bmw 318i, moving down the road

Question
I own a '84 BMW 318i, earlier this winter my heater core bit the dust.  I purchased a new core and valve and replaced.  Now when the car is at a stand still the heat produced is fine however once I start moving down the road cold air is all I get, this was not a problem with the old core. Any ideas on a possible cause and fix would be greatly appreciated.  

Answer
Does the heat go away when the car is moveing?  or when the engine is at more than an idle? (does heat stay on when you rev the engine with the car in park?)  it could be a blendoor, restriction or a nubmer of other things.  what happins when you turn recirculation on?
let me know
-cheers

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