G'day Norm,
You might want to run searches on a couple of the Oz 'lawnie' forums - seems that cable adjustment is a common cause, but that the gearboxes can fail too.
I'd suggest:
http://www.indmowing.com.au/forumhttp://lawnclub.com.au/forum If the site won't let a 'guest' run a search, it can be done via Google - '216 site:lawnclub.com.au/forum' as an example search term, without the quotes.
And gearboxes are available, but not real cheap - typical of contractor grade gear. Around $260 from the ODK Shop. Dunno about internal parts for those.