Thanks for posting this, Tjompen! I'm not alone!
I've been having the same issue with my K-1 and some Sigma lenses for a long time. All these lenses work perfect on my K-5 and *ist DS. Handheld or tripod, cold weather or hot weather, it always happens.
It rarely happened with a Sigma 105mm f/2.8 Macro lens I used to own, sometimes with a Sigma 24-60mm f/2.8 I sold, and frequently occurs with my Sigma 400mm f/5.6 APO Tele-Macro.
I really like this 400mm, but I have been considering replacing it with the D-FA 150-450 because of this issue. Any Pentax-label lens has worked perfectly, just Sigma giving me trouble. So, I'm very concerned when you write about the 150-450 having this issue too!
I've also tried mating that Sigma 400mm with a Kenko 1.4x teleconverter. The problem still exists with the TC attached, but not as frequently.
The camera loses connection to the lens in exactly the way you describe in the text, camera loses aperture value (F-- in viewfinder), AF stops working (camera shows MF) and whatever exposure I had ready to go is overridden by a different one (like 1/800s f/8 becoming 1/100s f--). I am shooting TAV mode for the most part, so the exposure is usually saved by the ISO switching. I end up with lots of blurry chickadees at ISO 100.
One partial solution I have discovered is this: If I hold down the lens release button very lightly, the lens will become fully functional as the locking pin is no longer making contact with the lens mount.. If you hold the button too hard, the camera AF screw (for screw drive lenses) will disengage from the lens completely.
Maybe you can try this too, you shouldn't lose AF because your lens is not screw-drive.
I think there may be some kind of intermittent electrical short involving the lens locking pin somewhere near the lens mount in the camera, or perhaps the lens draws too much power/camera can't supply enough power and it shuts down. You don't want any speculation, but from my testing I think one of these might be the case.
I won't send it in for repair because I've only encountered the problem with 3rd party lenses (Pentax will simply blame Sigma). If you're able to get yours repaired, please come back here and let me know what service to ask for!
Last edited by Mock; 05-19-2017 at 07:42 PM.