My pentax Kr started misbehaving recently, with the mirror or shutter or whatever going berserk. This happened just after I bought a second-hand tamron 24-75mm lense and a pentax 32mm lense for old film camera and started using them instead of the 18-55mm lense that came with the camera. So I figured the problem must be related to the new lenses (new to me). I carefully cleaned the silver ring on the camera (which is in contact with the lense) and did the same for the lenses, putting some dust on the sensor in the process (arghh!!). Since then, I have taken about 250 pics over a week, and the problem seems to have disappeared. So before you send your Kr for repairs, clean that ring and see whether it solves the problem.

[marcusBMG]

If you search the forum (and google) "mirror flopping" you will see numerours references to this type of problem AFAIK no-one has come up with a clear diagnosis. Some have suggested battery strength issues. IMO it is related to internal firmware glitches pertaining to use of vintage lenses with/without 1. electrical contact at the mount 2, aperture lever.