sounds like your starters tired fella
check the voltage to the starter when ign off and when the trying to crank
also check that you are getting a live when trying to start on the solinoid (sp)
have you tried to bridge the starter at all
also check the connections from the ignition barrel to the main loom as these tend to corriode and have a bad connection
my old fiesta did this and it was the connections from the black box to the main loom