I would do a compression test before I rebuilt that motor. 5500 hrs is a few, but I've seen them go a lot longer than that. Do you ever get the starter hanging up either stuck on or engaging the bendix and not rotating? If so, try replacing or cleaning up the starter contacts in the solenoid. Did that today on the 8650, and it starts MUCH better now, starter turns faster. Also, try eliminating all unnecessary loads during starting. things like the hitch bleeding off and causing the hyd pump to load the engine during starting can cause weird things.