I am so glad the nylon gears are ok
There is an oil hole that can get gunked up which can affect the motion of the fancy stitch selector (LH) lever.
So with the lid/top cover off, select the RH lever, and push it away from you as though to select the 1-13 stitch option.
You will see that the lever bar fits into a black V notch.
To the left and slightly in front of this V notch is the tiny oil hole. Shine a torch in and you might see it. If not then a few oil drops and a brush in the area and then more oil will help.
I found mine totally by accident by poking around with a cocktail stick, and the end of the cocktail stick snapped off, and got stuck, and it took a while to tweezer the splinters out, but it was how I discovered that there was actually an oil hole there.
The right hand lever should twang loudly when toggled between the two settings. If not, then more oil!
My 840 was rock solid for a week after I got it and oiling every day eventually sorted that out.
Edit - I just checked my 530-2 and sure enough that oil hole is marked in red... at the time I couldn't see it as it was so clogged.