If it's set up like I think it is, there is a larger horizontal screw that holds the Stitch Length Lever in place and puts tension on it to hold it where you set it. I have noticed on some machines that over time the little fingered washers under the screw loose their strength and compress making the lever a bit looser.
What I have done on a couple machines is to tighten the pivot screw a tad.
There is a small set screw on the bottom of the S.L.L. you'll have to loosen, then put a screwdriver in from the hole under the hand wheel which you'll need to remove first and tighten the big screw just a tad. It won't take much, you don't want it too tight. Then re-tighten the little set screw and test it.
My HOTHER S.L.L. creeps up and that would be the fix. It has a position lock so I haven't done it yet.
Joe