Heater intermittent - 2000 Caravan

The heater output on my wife's 2000 Caravan 3.8L is somewhat intermittent. The temperature gauge reads in the expected range but the air coming out the vents is sometimes cool. If you wait a few
minutes or soemtimes if you fiddle with the temperature control hotter air will again resume coming out the vents. I'm guessing there must be some mixer control that is acting up and am looking for suggestions on where I need to look for problems or other suggestions as to what the problem might be.
Thanks for your ideas.
John Keith snipped-for-privacy@juno.com
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