2004 F 350, diesel - so many posts, this might have been covered, if so bear with me. Heater problem, vehicle warms up, but when driveing off, will blow cold air, then blow hot and will cycle for some time before staying hot. Seems that when u stop it gets cold but as soon as u drive off again, gets hot

 

Check your coolant level. It sounds like you are low on collant. Do you hear a clicking sound? If you do, then the blend door motor is probably bad.

 

Thermostat not working - mine did this a few years back. The stat and the housing are a complete unit on the 6.0.

 

I just had a similar problem, i changed the tstat and flushed the radiator and also flushed the heater core seprately it kinda helped then i talked to a Frod service repa nd he told me that the heater core is probably blocked up, so i finally broke down and replaced the heater core and te peoblem is fixed. So what i am getting at is maybe you would want to flush the heater core it self and if that dosen't help well it's time for a heater core and with a little mechanical backround you can do it yourself it took me about 2hrs with a new rad flush.

 

Tks for the info, will try all and let u know