try the coolant temp sensor. if the starter and battery are fine then it sounds like the temp sender is playing up, telling the ecu that the engine is cold, and therefore the ecu is starting it up in coldstart mode. ive had this problem before on my zetec engined mk2, be worth changing the sender for the sake of a few quid!

mine would never start once warm, but would chug into life if i disconnected an injector.....then would run fine with it reconnected with the engine running! odd one!