heater blows cold air on 1999 Lincoln Navigator

We turn the heater on and it will blow hot air initially and then it turns cold....as we accelerate it will heat back up. This continues while we drive.

Asked by for the 1999 Lincoln Navigator
Sounds like an open Thermostat, pretty common. You may even have a Check Engine Light on and code for improper warm up, yes?

http://repairpal.com/check-engine-light here is an article explaining a bit about the check engine light and the Thermostat code http://repairpal.com/OBD-Code-P0128