Update. Checked and re-checked all the great suggestions provided here. Nothing found. Spent a good amount of time on the issue yesterday - ride hill outside house, wheel shifts, walk it back to garage, inspect. Repeat many times with wheel shifting EVERY time. For the life of me I do not see how the wheel can shift with it clamped down so hard! Then tried measuring whether wheel was centered and parallel to frame. My thought was that maybe the wheel is cocked when fully inserted into the dropout and it's just trying to right itself. Determined (don't ask how) that the non-cog side could be out from the bottom of the dropout a couple mm. Clamped it in and tested. No movement. Retested and retested - no movement. Went for a 30-mile ride trying to really work up hills - no movement. I have no reason to believe the frame or wheel is bent. As an engineer, this is bugging the heck out of me.