low heat at idle

Hi to all
have a 2001 montana with heat promblem has 68000 miles on it, When at low rpms it just blows out cool air no matter what setting i have it at, and when you rev the engine up it will blow good warm air, or when driving down the road, coolent does not seem to be low, and do not notice any coolent leaks,the oil is clean no sighn of coolent in oil, Has anybody had this promblem?and know the fix, or is this a known promblem with thease vans?

Thanks
Patrick