Not a cable or shifter issue.. I checked both. Not a constant problem with the rear derailleur.

BTW.. I have 2 sets of wheels, one has a Sram cassette and the other a Ultegra cassette... it happens with both.

Maybe this week, i'll take the derailleur off and go through it. I restore vintage VWs and build engines.. I think I can service the derailleur. But honestly... I keep the bike very clean.