That explains why my lens gave me much better results when I first bought it.
I don't understand why Pentax doesn't issue a recall on this lens. If there is a widespread problem with the lens, they should offer a free inspection and repair the lens if faults are found, similar to the Olympus recall.
Olympus - Service Announcement
But for all the posts in this and other forums, I wouldn't even have been aware of the problems my lens had. I was generally dissatisfied with the performance, even before I did the tests, mainly because my kit lens gave me better results. It certainly gives nowhere near the performance of my F*300 4.5, that has the potential to make everyday around the backyard shots into the likeness of magazine covers.
In any event, I have given the DA*16-50 in for repair, along with my other rogue lens - Sigma EX 14/2.8.