When I draw a spline in a sketch, the spline works great and I can drag it around as expected. i then use the spline to create solid geometry, no problem everything works great. Once I go back into that sketch and try to edit the spline however, the spline acts very erratic. i.e when I drag a point on the spline a very small distance, the distance the spline moves is huge, nowhere near what i moved my mouse. If I delete that spline and draw a new one in the same sketch, the spline behaves normally.
See attached video.
This happens all the time, on various parts, various features, even when it is just a sketch not related to solid features. I am using SW 2017 SP 3.0.
Do you by any chance have "Instant 3D" enabled?
What happens if you disable it and edit the sketch?