While in the distributor, grab the shaft and see how much side to side play there is.
Originally, I would set my timing and then find a week later it seemed to be 5 degrees different. Finally I realized the bushing was worn the shaft had way too much side to side play. The original distributors had 1 or 2 places to add oil to lube the shaft/bushing (little spring loaded caps cover them), which eventually wore due to lack of attention. I got another distributor and then could count on the timing. You can probably buy a bushing and replace it yourself