Batteries are brand new in the F90x, also I did try cleaning the F90x contacts (with DeOxit - which I use for audio connections). Cleaning contacts seemed to make a difference for a while, but the situation reverted in the morning (without dismounting the lens).
'tis a puzzle. I'm starting to regret my leap into modern technology (never had this problem with my Yashica FX-3 or FX-D !!)