I've got an 04 X Sedan with 85K. Heater not generating heat when in idle. Has been progressively worse the past 3 months. The A/C blower fans and controls seem to work. Heater/defrost blows air when car moving. Have had two opinions (1) water pump impeller deteriorating and not moving enough fluid and (2) air pocket in the heater core. Mechanic flushed heater. Hate to look at a new pump as it is a complex job. any ideas welcomed