The heat in the truck does not work well at idle speed when I'm stopped. However, I can place truck in neutral and rev the engine and get warm/hot again. I have backflushed the heater core and added new coolant. Could this be an indication the water pump is failing. Taking more RPM's and increasing pump head to get the water through the system. Or I guess maybe something with the heater core. It's a 2000 year truck. Any ideas?