Originally posted by SickEddy By the way the image in the viewfinder moves up slightly
This is the critical detail. AFAIK, the mirror is not supposed to move. All the green button does is let the lens aperture stop down so it can meter.
The focusing screen acts like a duplicate of your sensor. Light is supposed to travel the exact same distance to get to it as it does to get to the sensor. If the mirror moves slightly, that distance is no longer correct, and your focus will be off. It doesn't take much to throw your focus way off.
When I bought my K-r new in 2011, it had a faulty aperture control unit. The first symptoms were that the mirror would flip multiple times when taking a single picture, and sometimes it would fail to completely return to its rest position. Eventually, the camera would just flip the mirror over and over when I pressed any button. I sent it in for warranty repair, they replaced the aperture control unit, and it lived happily ever after.
I was one of many who had this issue, but I believe it almost always showed up very soon in the camera's life. How old is your K-r?
I believe the aperture arm, mirror, and shutter are all linked, so it might not be the mirror mechanism itself causing the problem. Something is amiss somewhere in those mechanisms.