I have a 99. F250 with 7.3 the other day i went to start it strated to turn over (starter hit flywheel started to turn then just not enough torque to turn over ) i waited 5 min tried again started fine next day same thing but would never work so i replaced both batteries same problem so then i replaced starter on first three tries same symptoms the she started and did fine for three weeks of daily use this morning same problem again stater spins hits flywheel begins to turn over. then runs out of stregth to turn motor over its not the selinoid as i have jumped that and it does the same thing please help going to rip new starter out but im sure when i get it to autzone it will spin and throw out but its not connected to a big diesel it has to turn over

 

I have had that happen on a 6.2l and the main problem turned out to be the battery cables

 

Check the connectors at the Batteries not just at the battery post but where the wire hook in I have see where they get corroded also if they have ever been replaced with the ones that have the two little bolts that secure the wire to the terminal they are famous for getting corrosion. Note it is also possible for the starter to fail we were working on a Lexus LS400 the starter would hit and start the other times it would just spin took it to AZ they check it and said it was fine! Told them to check it again (You just do not pull a starter from a LS400 located top left side of the engine) they tested it three more times failed on the third test! Also they can not check for a weak Bendix.