abclarke wrote:
Sounds like a similar problem i had, i just got a new starter off ebay and problem solved

x2, sound like the starter motor gear is sometimes not engaging fully on the flywheel and sometimes not disengaging properly.
A new starter should fix that (if flywheel gear is ok), or you could try taking the starter apart and re-greasing the sliding parts. Water/mud can get in there and gum it all up.