Check the slot on the Dist gear. There are some in the aftermarket that the slot where the Dist shaft engages that are "fishmouthed" from the manufacturer that will do exactly what you described. It should fit snug not loose like they are being made. Had this problem where the Holley dist would keep loosing or fall out of phase. Put a light on it and it would just keep moving around. You can call me with any questions.
Todd