I had just the same trouble on my Rover-engined 1999 +4.
It turned out to be the starter motor in the end, which was drawing too many amps apparently. I changed it for a 'High Torque' PowerLite unit from Eurolec Components at Brierley Hill near Dudley (at a cost of £130 including the mounting adaptor)and since then it's been no trouble at all and spins over a treat.
