my F250 HD
 

My f250 is boiling back thru the reserve box changed the thermostat never had a problem with this before.It only does it after i drive it a little.What could it be.

Poss clogged radiator or engine problem (head gasket etc). I'd start with the radiator.

Id start with a hydrocarbon test of the radiator before I did much. if it dont take long for it to over pressure and the gauge never gets hot you have compression in the coolent/bad headgasket.

If you have a Ford with a single row radiator, that is most likely the first problem. They clog some at the bottom of the rows, causing heat to rise in the engine, then that is when the head gaskets really start to give up. The dual row ones have a better chance of not overheating, because of the finning and dual rows. Something to think about.

Do the compression gas test as Jet said, but remember that these radiators are now 17-20 years old. It don't take much to ruin an engine with over heating, and the radiators are usually overlooked when doing engine repairs.