I have a K20D and have exactly the same problem with both my lenses (a cheap 18-55 that came with it and the expensive IF 200 F2.8) I bought about a month later. I contacted Pentax, and they told me to send it in. It's under warrenty, but to tell you the truth, I don't trust them. They already screwed me out of a measley $100 rebate on some silly technocallity, and I'm afraid that if I send it in, they'll keep it for a long time (I can't afford to be without it for long) and then tell me I have to pay to have it fixed because of some small print in the warrenty.
I can still get great shots with MF with my 200mm, but the cheap shorter lens is clunky and unreliable on manual. Nevertheless, I don't think I'll send it to them, and I don't think my next camera will be a Pentax. Great camera and lens, poor company.