2000 Ranger heater problem??

My wife has a 2000 ranger which has been relativley reliable except for the heater this winter, In very cold mornings or nites in the single digits the heat only blows cold air period, The temp gauge is at about the half way point which is normally where it always is and the heater hoses get hot and thermostat seems to be opening and closing properly by the pressure on the top hose, I can hear the vacuum door opening and closing when I turn the switch on & off but no heat? this morning she gets no heat so she uses my truck & I deceide to take hers to work and check it out, I work 2nd, So I leave for work & its about 18 degrees out & guess what plenty of heat in 5 minutes?? But tonite when I leave work at midnite 6 degrees? No heat again?? Any help or direction to follow? Thanks

Todd, given that you do have heat when ambient temps are higher than the teens, It seems your HVAC system is fine. I would still suspect a bad thermostat. The dash temp gauges are almost useless for troubleshooting a bad T-stat.
G/luck
Joel

Well with a little investigating it turns out there was some moisture in the box & with the single digit temps it was feezing the blend door in place until the heat would thaw it out, But only with the fan off, In cold temps with the fan on at engine start there was no way to thaw it out, After I drained the box & dried it I have been in good shape, Thanks