Delco distributor problem.

I am using a delco window type distributor on a 1961 Studebaker.
My dwell is set correctly.
When I tried to set the timing, the timing mark fluctuates a half inch or little more.
Then I put in another distributor and now I don't have the problem.
Did I have an electrical or a mechanical problem
on the window distributor?
I would like to fix that problem because on the window type distributor it is easier to change and set the points.
Thanks
Paul