'91 300SE. Old Pearl is freezing me out in this Dallas cold snap!

I seem to have the opposite problem from all the heater posts I researched:

My heater works just fine around town; blowing warm air from the proper vents, responding to temp change requests, and maintaining a set temp perfectly.
I can turn the wheel all the way up and it gets pretty warm, along with increased fan speed. Seems to be working just as it should.

BUT.... On the freeway, the air temp starts to drop. Not all the way to outside temp, but maybe 60 degree air in 30-degree weather - Brrrrrrrrr.
If I turn the wheel all the way up, the fan increases but the air temp does not.

I checked:

1. My auxilliary water pump has been disconected for 5-6 years with no problems. But the classic issue there is poor low-speed heat, correct?
2. Engine temp is good = 85C.
3. Monovalve clicks and modulates in the garage - seems okay.
4. In-cabin sensor seems okay; I installed a new suction fan last summer, and the temp stays right on around town.
5. All the vacuum pods are working; all the vent settings do the right thing.
6. Haven't checked the heat exchanger temp sensor; but the system works great around town.

I'm thinking maybe an intermittant monovalve? Maybe the increased water pressure at high speeds forces it closed or something?

Any thoughts?

Cold in Texas,
DG