I'm not sure what is causing your exact issue. These lenses aren't quick shift and it isn't good to try mf when the camera is set to AF. Take it off the camera and operate the mf ring. Put it back on the camera and operate the lens in af for a while. Set the camera to mf and then try to mf. However, it sounds as if this lens may need to go for a service. You may consider emailing Eric Hendrickson and see if he will take a look at it.
http://pentaxs.com/
He generally doesn't like to do AF lenses, but this one he may consider but you will have to email him first.