Thanks for the suggestions.
I have already being doing a variant of re-positioning the DEC wheel. At the end of each imaging run I have SGP park the scope in position #3. For the next evening's imaging session I loosen the clutches and manually move the mount to park #1, re-tighten the clutches, power up and do a star sync to get going. I would say over the last couple of weeks that I have moved both the DEC and RA worm positions through a wide range of different starting positions.
I have also already removed the DEC motor cover and verified I can move the cogs easily by hand. In fact I actually stopped tracking at a point where I was seeing the oscillation, disconnected the motors cable, and verified I could move the gears, and I couldn't detect and tight spot.
I will do as you suggest and do a full "overhaul" on the DEC worm/wheel. I've never actually re-greased the worm or wheel (I bought the mount second hand several years back and it's originally from 2006). And I will do a re-mesh.
I'll report back later on whether that helps.