'99 Cummins ISB Diesel engine won't start.

My 2001 dodge 1 ton diesel had a starting problem. It stated fine when the engine was cold but after driving and the engine got hot it would turn over (crank) but not start. A diesel engine has to turn over to a certain RPM to start. One dealership told me it was weak batteries (I have 2ea). Replaced the batteries, same problem. Another shop told me the injector pump may be going bad. The shop that finally figured out the problem found the starter was dragging just enough to keep the necessary RPM from being achieved and starting. Replaced the starter and problem solved.