Climate Control Behavior

[alexl]

My Climate control in the 1982 300TD is acting up. sequence of events below:
1. First 5 minutes of drive, everything works as expected. Get heat out of vents on any setting.
2. After 5 minutes, the blower fan turns off on automatic setting or eco mode. Fan buttons, temperature control do nothing. The blower fan turns back on if i switch to defroster, but there is 0 heat, no matter the temp setting.

I have: confirmed Monovalve function, bypassed aux pump, confirmed full of coolant, confirmed coolant operating temperature. I am confused that it consistently works for a few minutes and suddenly the blower turns off and does not come back on unless defroster is active.