You have a loose connection. it is either on your battery, or right on the starter where the power wire connects. ford's are notorious for the starter wiring getting some corrosion. once you find out if the battery cables are loose, remove them, then crawl underneath and remove the wires to the starter and inspect them for corrosion. a wire brush will be sufficient to scuff the surface up a bit, then make sure everything is nice and tight. this should solve the issue.

9 times out of 10 its a loose ground wire on the battery or battery terminal corrosion.