OK I have tried everything I can think of. Air diffuser works, evap fan works, ive replaced the defrost and defrost timer but still the same. The one interesting this is now I hear a intermitent buzzing sound, its about 5 seconds long from either back side left or right where the evap coil and fan are. I can't pin point it. It comes and goes with out any known pattern. its not the compressor

I notice the evap coil will have a thin later of frost but it won't ice over. The defrost heater works. I am out of idea's now on what to do.

So its not:
the air diffuser,
evap fan, or
defrost.

thx