I have similar experience, I have sent my 16-50 to Pentax Germany twice, first because it had a wobble issue, second time because of SDM failure. Both times my lens came back with a notice, there is nothing wrong with the lens.
I stopped using the lens for many months, it was too annoying to always live in the uncertainty if the SDM would work or not.
Few weeks back I tried the lens again, now the SDM does not work at all anymore, it fails totally. I now have an excellent manual focus 16-50mm Pentax lens. Better almost than having to fear constantly for the random failure! I kind a like this lens again. It's IQ is still superb!
Originally posted by habsfanusa Hello all !
i just heard back from Pentax - WONDERFUL news, i'm sure everyone will be relieved:
"there is no known issue with the DA* 16-50mm SDM lens at this time".
So, there you have it, right from the source, official pronouncement, June 21, 2010, 11:21 AM.
Now we can all sleep so much more soundly.
I will be posting a thread of my correspondence w/ Pentax on this Forum later this week, but in the meantime I just couldn't wait to share.