hi Mark
This is a common problem as our cars age, the odometer gear concerned will be very brittle now and usually fails as you have described after resetting the meter. I had to fix my own last year....you need to identify which gear you have although iirc about 95% of cars have the same gear....I had to order my replacement from the states as I failed to find anyone UK based who supplies these, others may know better....this link my help with what's involved...
http://www.odometergears.com/documentat ... Repair.pdfiirc I bought my replacement gear from eBay..
regards
Pete