Well the next thing would be to take the starter off, pull it appart and inspect. Get some good electrical cleaner and clean all the armiture and brushes. Also check the seals to make sure that you do not have primary fluid getting into it (was one of the 05 issues, not heard too much on 06 though).
Is it still under warranty by chance. If so, hate to say it (for you know it will be a hassle) but see if they can get to it quick. If not, then pull the starter and see if you can figure out what the hell is going on.
You know you may also want to check the drive section inside the primary and make sure it isd not binding or damaged on the ring or gear.
Last, if still un-sure and before what IMHO is way too dam much money for a starter (part of the nature of the beast for MC in general), maybe see if there is an old time alternator/starter shop near-by and see if they will take a look for a few bucks. Maybe something simple that they could fix and though it has been awhile, they used to be a great way to save some money and get things right.
Good luck and keep us informed.