Hard to tell based on limited information, but....

1. check to see if engine temperature gets up to normal (85 - 90 C)...if not thermostat may be bad. The erattic behavior is also consistant with a thermostat going bad.

2. In your 82, I think you have the automatic climat control, but I dont know if the same as my 85 300D....but probably similar...could be inoperable "mono-valve"...this controls flow of hot coolant to heater coil, controlled by signals from push button climate control unit. Rebuild kit costs about $35 and takes about a half hour to complete.

3. Malfunctioning pushbutton control unit....cant be repaired by DYer if this is the case....rebuilds cost about $100+

good luck,

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)