The heater core is restricted. Increased engine rpm means increases pressure and increased flow thru the heater core. Start by removing the heater core hoses at the intake manifold and radiator, hang them over the fender into a suitable catch can (large drain pan or 5 gal buckets) and flush with a garden hose. Do this alternating between both hoses being either the drain or supply from garden hose.
Note: the hose removed from the intake manifold is the supply to the heater core and the hose from the radiator is the return from the core back to the radiator. So start with the supply line and then move to the return line. Using the return line is back flushing the core and may help clear out and clogs.
Flush until the water is as clean as you can get it. Report back with your results.
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