If the sight is jumping then the coil spring inside the slide is either weak, missing, broken, or the retaining nut is partially unscrewed (common).

Other possibilities: rack-and-pinion mechanism clogged with dirt. Unscrew eyepiece, remove slide, drop in can of mineral spirits for a day or two or use ultrasonic machine if you have one. Brush with M-16 cleaning 'toothbrush'.

Substitute known good slide, if you have one.