Loss of heat on 98 Taurus

[jimh123]

My engine temperature gauge never goes past he half way mark.
When it warms up, my heat will be warm for less than a minute
Then starts to blow cooler air, if it idles for a minute it will blow
Cold air. I have replace the thermostat and the fluid seems to be at the right Level.

Any help would be very much appreciated.

[denisb]

Could be an air lock in the system.