No heat at high RPMs

Here's what happens: slowly going down the road at 1400 rpm, blowing hot air. Revving it up to 1700rpm, in a matter of 10 seconds heat transitions from hot to cold. Dropping RPMs back to 1400rpm, no change. Pressing clutch, dropping RPM to 600, air goes back to hot in few seconds. During this entire time heater core bypass vent door stays closed, coolant valve voltage jumps back and forth between 6V and 8V, coolant temperature doesn't change. The only explanation I have, the coolant flow to the heater core gets cut-off somewhere else. And the driver says it still blows hot in the bunk. Is there another valve between the T-connectors where it splits and heater core. Anybody has coolant flow diagrams for Volvo?

 

Thanks, I will try that.