Code reader is a good starter. Heat can cause a myriad of problems, electrical, mechanical, fluid related. The reader might help pinpoint the sources easier. One that can do reading whilst out on a run will enable monitoring at increasing temperature. That, at least will cover possible sensor issues. As temp goes up things expand so leaks might be created, air, fuel, vacuum etc.. Be methodical and thorough checking one system at a time.
Try some of the Ford owners forums too. Even kitcar forums as the Zetec was a popular engine.
Good luck!
Just had a thought.......how are your plug leads? Old? Replace them. Quite a common cause of bad running. I had an escort that spluttered. New leads cured it.

Last edited by sospan; 18/03/17 12:22 PM.
Plus Four MY23 Furka Rouge