OK so I contacted Pentax/Ricoh Germany and they offer service free of charge via the local repair shop. Indeed they've done it: only some adjustments in the AF and mirror plus some other things, nothing changed/replaced. Now it seems to work correctly again, hopefully for some time. Thanks for your contributions.

A funny thing found during this weekend: the strange behaviour with shutter releasing several times by itself before finally reacting correctly, is NOT present when in manual focus! When in AF it does it (randomly). Any clues?
Anyway, contacting Pentax (and hoping for very nice response) is I think now the only solution.

Sure that's what I will try, replacement or repair for free or a modest sum. We'll see. I'll let you know the outcome.

I'd do it like you but unfortunately my camera is not under warranty anymore. Otherwise I would not bother you with these questions, of course.
I'll contact local Pentax dealer to see what can be done and especially for which price. But thanks anyway.

No, randomly. I have no idea whether it is related to outside temperature. Usually upon turning on the first time in the day there are no signs of problem but after several shots it starts.

Ok after several weeks I haven't observed anything new, battery (original) full or half empty, different SDHC cards, different (Pentax and third party) lenses. Sometimes it simply flips the mirror several times and only after that allows shooting or focusing. Latest firmware, I forgot to say, mostly default settings.
It is not the behaviour observed by some users with the mirror going mad. It is only a few flips, say 4 to 5.
The only thing I noticed is that when cleaning the mirror there is some whinning sound but it could have been there before as well.
Anyway, thanks for your comments.
I'll see whether Pentax can do something about it.

Yes genuine battery. Will do the reset and then upgrade. Hopefully I just activate some odd feature. Thanks for your help.

Hello, I have a strange problem with my K5. Sometimes when I turn the camera on it flips the mirror without any reason, sometimes several times. On the same occasion (or another time) when half-pressing the shutter release button it flips the mirror several times and only after that I can take an image. Several times is about 2-3-4 times. Firmware 1.15, will be upgraded to 1.16 but this does not seem to be the issue. All settings are normal (mostly the green ones or default ones), camera operated almost exclusively in Av mode, happens with different lenses (third party at least, will have to verify with Pentax ones). Before the camera worked properly. Shooting PEF with center AF and AF.S mostly.
Any idea what could this be? I can still take photos but it is quite annoying.
Thanks, Jiri

Well I just meant that Pentax wouldn't want their users to come back to normal AF, once the SDM was implemented. And of course they cannot admit they have problems with the SDM - well they can but don't want to. Which yields the same result, probably even worse than if they'd admit it.

Why should screw driven SDM lens focus faster than a non-SDM lens? Is the internal mechanics so different (less friction or whatever)?

When inventing SDM why should anybody switch back to normal AF? No wonder that Pentax states nothing about this in the manual.

Thanks for the answers. I asked about the price of reparation and it would cost me about the same as the price of the camera (250 ChF) or even more so I let go. I'll perhaps wait for the Tamron equivalent without SDM (and its problems).

Btw on the pentaxforum there was a person that seemed to know something about disabling the SDM. He seemed to known quite a lot about the firmwares in general but unfortunately there was no simple guide as how to disable the SDM.

That said, I'm looking forward to a sunny weekend with all the autumn colours in the forest!