Have you had a look inside your thermostat housing to see if it still has the plastic float that is there to help with air in the system?
My brother has been having similar problems and found a thread on this site somwhere explaining about the float, when he took his housing off his float was missing, so he bought a new housing from ford and the car seems to be a lot better now.

I'm going out now but when i get back i will try to find the thread and post a link up!!!!