Maintenance Issue with my Singer 15-91

Roughly four years ago, my friend Rain restored my vintage Singer 15-91.

Just to give you a sense of what it used to look like (below):



Would you believe it's the same machine?



Rain did a marvelous restoration job both inside and out, but one problem has surfaced: after a few projects, the stitch length lever begins to loosen until finally it moves up and down on its own, making it impossible to maintain a single stitch length.

I've managed to tighten the lever by removing the balance wheel and motor and tightening the screw holding the lever in place, but -- perhaps I haven't tightened it correctly -- after repeated use the lever loosens again.

Today, I tightened it once more only differently this time: I moved the lever up and down as I was tightening the screw and I seem to have found a place where screw will move but the lever won't loosen. We'll see, over time, if this proves successful.

It's not hard to remove the balance wheel and motor -- it all explained in a PDF I downloaded of the original manual.











This is the screw I tightened (below):



From the side, the whole mechanism looks like this.  The bit of the shiny screw is visible on the far left.




Removed from the housing, it would look like this:



I don't have the skill to take the whole machine apart and I'm hoping that's not what's needed.

I'm crossing my fingers.  I haven't heard from Rain in years and I'm not sure if he's still performing sewing machine maintenance (I believe he had recently taken Ray White's sewing machine maintenance class around the time he helped me).

I love my Singer 15-91 -- one of my early vintage sewing machine purchases -- and hope to have a long future with her.  In the worst case, I can just keep on tightening when necessary, or use her only for buttonholes (which wouldn't involve changing the stitch length) though that would be a shame.

Has this ever happened to any of you?  Any insights/ideas?

Have a great day, everybody!