Seem to have developed a problem with the heater on my Fusion.

Providing the engine is doing at least 2000 RPM the heater works fine, however, when the revs drop below this (stopping at lights or slow moving traffic) the heater only blows cold air (not the best time of the year), it then takes a fair while to start blowing warm/hot air again.

Blower works fine, coolant level is correct & Temp Gauge is in normal range & doesn't drop at all.

Any advice would be appreciated.