Heater blows cold under 1500 RPM

[canoecraft]

On my 2009 Saturn Vue 4-cylinder, the heater works great, but only when engine RPM is over 1500. At highway speeds, heater works normally blowing hot air, but when idling under 1500 RPM, the heater just blows cold air. In cold weather, driving at highway speed, heater blows hot; if I come to a red light, within about 10 seconds of stopping, the heater blows cold while idling. Resuming highway speed returns the heater to blowing hot air. On the highway, the coolant temp runs from 195-210 degrees.

We've drained the coolant and done a complete system flush including the heater core. No difference. Any suggestions on possible causes or solutions?