If you wired the valve to a switch, you would have full flow to the heater core no matter what when its turned on. You could turn the a/c on and mix the hot/cold air. You could even turn the pushbutton unit off and leave the valve on, and you would still have warm air come through the vents at highway speed. But this isn't really fixing the problem.

I was just thinking that maybe something is getting stuck when you reach the higher cruise rpm's, because there's higher water pressure there than at idle. But it still doesn't make sense. You would think something would get stuck and stay stuck, not go away at idle.

__________________
Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C