Originally posted by Nickrs Before I send it back to Pentax has anybody any insights into what has happened?
What you describe happens sometimes. Conventional wisdom is to consider a power issue first. A low or compromised (unable to support power draw at full voltage) battery is a common cause. One of the benefits of having one or more extra batteries is that one may swap out the suspect battery to see if the problem goes away.* The second common cause is water incursion. A third is a main board or other electronic failure. Except where the battery is the cause, these things typically don't go away by themselves or self-repair.
As for previous models with shutter or mirror "stutters", there have been defects in various models at various times, but not quite like you describe. The closest might be the runaway mirror syndrome that happened with early K-3 production, though in that case the mirror would cycle wildly, but the shutter did not.
Steve
* I have three batteries (the original Pentax that came with the camera and two Wasabi-brand) that I use in rotation. The camera always has a battery and I always have at least one backup.