300SDL

Wrestling with no heat at highway speeds. Just installed new monovalve. Did the test suggested by the venerable ah-kay, to wit:

1. Unplug the connector to the monovalve. This will cause the plunger to retract and call for heat ALL the time.
My result: heat all the time, until highway speeds. As speed increases, temp decreases to no heat.

2. Then put a constant 12v on the monovalve such that the plunger will be fully extended to block the coolant and there will be NO heat.
My result: no heat ever.

I also tested the new monovalve before installing it and it operated perfectly. BTW, cooling system is flushed and clean with proper fluid.

So if the monovalve is 100% open as in test 1, is something keeping the coolant from getting to the heater core at high speeds? Is the only reason that there is heat at idle and low speeds because of the aux pump? Is it possible that the valve(not the cartridge part that gets rebuilt) at the bottom of the monovalve chassis is blocking the flow at higher speeds?

Thanks for looking.