A couple of months ago I had an NOS 2403 on my Sun machine. Brand new never run distributor. did not come up to specification. I had to take it completely apart, clean it from top to bottom, polish the advance weights and relubricate all the moving parts , before it would come up to specification.
After all these years, your distributor may be at the end of its service life. Time to check the condition of the gear, and the advance mechanism. Also look carefully at the lobes for really rough, rusty or uneven wear.
Finally, make sure there is not excessive clearance between the top of the gear and the bottom of the distributor casting. To much clearance can allow the shaft to move up and down changing the dwell and point clearance.
