I have a JCB 506C telehandler and after being started twice in short time it will slowly turn over with the key. If you leave the key off and jump the starter the engine will spin fine. I do not have any tech manual son just looking for direction to what valve to look at because I feel I have a valve not dumping to sump on start up and causing a hydraulic lock
Sounds like you have ignition wire that might be breaking down with heat from being cranked over or key switch might be going bad not allowing full amps to full .What is the serial number on your unit?
Check earth strap on battery to chassis and if you have an isolator on the battery lead as well bridge this out and try it ,try looking at where the wire goes onto the starter motor for good connections and the trigger wire on the solenoid is in good condition also. Also check that the ign switch function is correct and you are getting battery voltage on the wire to solenoid on starter i think you will have a neutral start relay under the dash or to the right of the steering wheel under the flip up fuse / relay panel may be faulty