Is the throttle shaft wiggly?
That causes an intermittent vacuum leak.
Sticky advance weights, rusted plate.
I make my own bushings for my MSD distributor for instance and the metal I use which is basically a little nut I grind down into a egg shape rust's overtime if you don't cover it in a little bit of light grease or oil dot-dot-dot next thing you know the things stuck with initial timing or even sometimes it would stick at full advance 24/7
