Please don't miss understand, I'm not trying to challenge or argue with any one, simply trying to clarify so as to not confuse OP.
IMHO, while to much play in the distributor may cause irratic time (jumping up & down), only a few things can cause the timing to creep (advance). One would be something inside sticking and not returning completely as I mentioned earlier (advance plate sticking), or possibly something external influencing the distributor base. And why I mentioned base shims, if the distributors shaft does not engage the oil pump correctly (slightly too long, it happens, why they make shims), as the motor heats up it could move the distributor slightly. Of course eventually if not corrected it will damage the gears. Just saying it's worth a check.