are you using a genuine ford fan switch or a pattern one?
it sounds as though the switch is sticking open as soon as it gets to temperature & not shutting down again. what does the dash board temp guage read? does this go down when the fans come on?
other possibility is an airlock. try running the car with the header tank cap off for a while to get rid of any airlocks.
also what make is the rad? there have been a few cases of poor quallity rads not flowing properly