Look, again, I am far from being really qualified for this, but, other than making sure that the speedo shaft is in one piece and properly lubricated, I calmed down my speedo by tightenieng the special nut on the gear box rear shaft. When this nut is loose, the speedo drive is not driven properly and this may be causing erratic speedo readings. This nut causes the speedo being driven just by its tightness.
It is described in the Technical Manual
https://www.sl113.org/wiki/Electrical/Speedo#ErraticSpeedoA lot is also described in the threads. I think someone (DrBenz?) desribed a simple test of this nut - to see if it is tightened - and again if I remember correctly, check it maybe - it was about sticking a flat screwdriver in the place where speedo shaft goes in the back of the gear box and try to turn it. If you can turn it - the nut is loose for sure. Please verify this.
I had very similar symptoms and the cause was this nut not being tight. I do not remember if I just had to remove flex disc to fix it or I did that when I had shaft removed. But I think if you have this big spanner for the sleeve nut, you can unscrew it and move the shaft backward to access the nut that needs to be tightened. Again, if I recall correctly, I did it 3 years ago.