Every now and then, my 6-speed slips. I can't be sure it's linked to a particular gear, but it happened in third, and also fifth and maybe sixth. It's not even a 1/4 turn of the pedals, but it is noticeable. Is this a sign of a hub that needs replacing or one that needs servicing? For the record I've done just over 5,000 miles on this bike in the last two years.