For about a month now my mustang will randomly blow stone cold air out of the heat. It will only do it if I've been driving it a few minutes then it sits and idles while warming up. But once I push it above 2500 RPM or above 40 mph it will go back to blowing out burning hot air.

Coolant was just put in and tests at -45. I was thinking maybe the thermostat was getting stuck but not sure. Anyone else have any ideas?

 

Usually happens with low coolant so since that is good and works if pushed I agree with your therm deduction.