Here's a new input to an old thread. I hope it's still timely.
Try putting an tin foil where the contact pins are. It solved my problem.
I've been having the same mirror flop issue with my manual focus lenses. The mirror sometimes flop and refuses to come down completely after an optical preview or exposure reading. Sometimes the camera just simply lock up. I sent it back twice to fix and both times it came back, the technician insisted that there was nothing wrong with it. So I suspected it may be caused by the lens itself.
If you noticed, the mirror and aperture can only function correctly if all the contacts are fully shorted. Sometimes there is wear on the mount where the contact points touch, so the short is intermittent. I thought maybe the lens mount is not fully shorting the contacts so I reinforced the connection by sandwiching a tin foil between the contact points and the mount. Bingo, no more mirror flop!
The perfect explanation would be that not all the spring loaded contact pins may not touch the lens mount. The metal lens mount will function as a common point to short them out. So stuffing a conductive foil in between improves the contact. Cleaning the lens mount and contact point will help too, but stuffing a foil in between is better. Or else, use AA batteries.
I still wonder why the mirror flop does not happen with AA batteries. I hope my next Pentax camera would not require me to tinker and hack to work properly