Qksilver,
I checked out your YouTube video- although nowhere near as bad or as consistent, my 50-135 is doing the same thing. The one thing I especially picked up on was the squeaking sound your 16-50 is making. The 50-135 is starting to make the same noise.
I'm sending the lens in to Pentax -today- BUT, it's a month and a half past warranty.
Optically it still makes great images, but...
When I first bought my K10D, over a year ago, I was really interested in other options in lenses. The Sigma 50-150/2.8 sounded interesting, but not available for Pentax. The Tamron 17-50/2.8 had a great reputation, but not available for Pentax. So I spent big $$ for Pentax's two top of the line zooms, the DA*16-50 and DA*50-135. The 16-50 was one of the bad ones and had to be replaced under warranty (the replacement -IS- a very good copy though). And now the 50-135 is going bad. Ironically, those Sigma and Tamron lenses I was looking into before are NOW available for Pentax, too.
I hope it doesn't cost me an arm and a leg to get this lens fixed.
Paul