do use your SDM equipment regularly ...
I once (about 1,5 years ago) had one of my DA* zooms (either the DA*16-50 or the DA*50-135, don't remember which) turn a bit unresponsive on me. It was indeed the first time I used it after a relatively long period of using mainly the limiteds. Manually turning the ring a few times solved this like it did in your case.
Since then I use them more frequently as I'm doing more family and event photography and using my favourite primes is just not the best idea in those circumstances. I haven't had any of the two DA* zooms giving me any problems since, SDM continued working as if new until the present.
Fyi, both my DA* zooms are early production copies purchased in August 2007. They did spend 15 minutes in service early 2008 because of focus accuracy problems on my K10D (that was SDM ready before any production SDM lenses existed, so couldn't be calibrated in factory), though I believe the camera's SDM was calibrated to the lenses and not the other way around. Note that I didn't get clear information on what the service man actually did.
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