Why does the car heater stop blowing warm air at a stop and warm up slowly?

I suspect it coulkd be simpley a stuck open thermostat. Most Fords the temp guage at full operating tempuature the guage should indicate appox. 2/3s on its scale. If the guage is indicating lower than it should the heat will be low also. Replace thermostat with a new one if this appears to be the case. If that appears to be ok the other common issue we see is a clogged heater core. Sometimes flushing the heater core will help this and in worst cases the heater core needs to be replaced. A good technician can determine this for you.