'90 V8 Quattro heater fan mostly not working

The high tomorrow is 5F here in St. Paul and the fan of my heating system
only works intermittantly. It usually runs when I first start the car
totally cold in the morning, but it shuts itself off after about 15 minutes
even though the interior temperature is no where near the set point. After
this, the fan won't come on for the rest of the day, except for very
occasional and too brief moments.

I suspect the car thermostat is stuck open since the water temperature stays
around 60-70C and I know the fan isn't supposed to come on until the water
gets hot. However, the "high" setting on the heater fan is supposed to
override this and the fan does work when the car is first started and very
cold.

Another clue is that the fan worked okay until the outdoor temperature
dropped below 25F as it did recently (and will stay for the next few
months).

I'm going to have the coolant thermostat checked out soon, but I'm not
convinced that this is the problem.

Any suggestions?

Thanks,
Tom


PS: My first car, a 1961 Buick LeSabre, had chronic problems with the
heater. I finally replaced the temperature control with a threaded pipe
water valve in the heater hose and a toggle switch under the dash to control
the fan. Kind of crude, but it was foolproof.