528e strange heater problem

my 1988 528e recently began blowing cold air from heater above 1800-1900 rpm. Below 1800-1900 rpm the heater blows hot air. i've replaced the thermostat, and the problem persists even if the wire is removed from the heater control valve. HELP!!!! only other thing i can imagine is an internally-collapsing heater hose at higher pressure/rpm any suggestions??