F-250 Ford 6.0 desiel after the pickup is warmed up and the heater is on it blowes cold air when the pickup is adle and hot air when it is revved up?? I have changed the thermostat and the in line switch that is in the heater hose. But still don't work?? What to try next?

Focus on the blend door actuator. If you have Electronic Air Temperature Control (EATC), run the diagnostics and record any codes. Afterwards, turn the system off, without saving any codes. This may clear any non-fault codes and allow everything to work normally.

Try back flushing the heater core. Sometimes that helps. But do not flush engine.