Sounds like the resistor network for the blower is bad, or the blower itself isn't working. It will run down to low speed, then come back up as needed. It sounds like your's is simply shutting the fan off -- if the high resistance coil (low speed) is bad, no current to the blower.

Do you get heat if you turn the temp control all the way up, but not to the final "click" that turns on full heat no matter what? If not, I'd suspect either some bad switchover valves (behind the PBCU) or bad servos. The valves switch the air flow around to the appropriate vents (heat to the floor, cool air out the side and center vents, and defrost to the windshield), and if you have a broken vac line or a dead servo, the air may not be going where it is supposed to.

Only way to check is get a hand vac pump and locate the servos and test them. They must hold vac indefinitely.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!