I have now had the same problem with both my Bronica SQ and my Mamiya RZ: a roll of film will have several good exposures and a bunch of wildly underexposed frames. There didn't seem to be any rhyme or reason to it until I studied the 8 rolls I processed last night and found that the first frame of every roll was good, followed by however many of the same subject that were underexposed. Whenever I found a new subject (which might take ten minutes or an hour), the same pattern repeated itself: one good frame, the rest bad. Obviously, the pause refreshes, to coin a phrase... The battery checked out okay, and viewing the shutter speeds on the RZ through the lens also checked out okay. Then I remembered that, even though even though the old battery read fine on the status light, the problem on the SQ only cleared up when I installed a new battery. Viewing through the lens on the SQ, it was very apparent that the shutter was wildly inconsistent. So, thinking battery problems, I dug around and found this, which was pulled from the now extinct Bronica website via the Wayback Machine: Note that Bronica does not recommend the use of Lithium batteries. So, I dug out my SQA, which I wore out last year without a single problem, and naturally I had replaced the previous battery with an alkaline cell. And in my RZ, which spent about a year idle, was a lithium. I'll be replacing all the lithium batteries in my SQ and RZ, and I'll be sure to report back if I have any further problems. I hope this thread will pre-empt anyone else blowing a bunch of good film on bad exposures!