i had an issue with my F2 that might have been the same thing: severe random backfocus, just like your first sample, one in every six-or seven on average, say, five or six per roll, focus several metres behind what was sharp on the screen (and i was super-careful after the first few). i'm not sure it was the lens as i, ahem, focused on the body: my local nikon guru saw the prints and didn't even want to look at the lens, he nodded, smiled and kept the body only... and he seems to have smiled with a good reason as the problem hasn't reappeared yet in a couple of years
best of luck in your search--i still remember how annoying it was!
ps. just remembered--i think the whole thing started after i tried to fit in an F3 screen (as i had done many times before); this one somehow started doing this, as if the ground glass were loose in the frame??? but then again, i put F2 screens back in and the issue was there... hmmm... i know, useless...
Last edited by Vilk; 09-16-2013 at 10:59 AM. Click to view previous post history.