I had this issue with my 16-85 lens when the aperture hung up internally. Because it varied with exposure, it was a bit hard to trap at first. I observed it, figured it out, felt a little glitch in the lever operation, perhaps a bit of grit, and then after manipulating the lever, it went away and never happened again. Spoiled a bunch of pictures - and of course they were pictures I took in Bhutan, where I am sure never to go again....
One thing you might try is different exposures of the same thing. If it's an aperture lever issue, exposures meant to be taken wide open will be correctly exposed, but those meant to be taken stopped down will be blown in increasing degrees.