If the auxiliary pump is blocking flow, it won't matter. The monovalve is blocking the same flow when it's energized. I would suspect a low coolant level where the pump does not have the head pressure at idle to circulate coolant to the radiator, it's simply circulating through the bypass circuit. A slipping or inoperative WP will cause the engine to heat under load due to lack of coolant circulation. A partially stuck thermostat will do the same. The only other thing that will cause heating at idle speed is poor air flow, and it sounds like that scenario has already been eliminated.

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'83 300D, 126K miles.