Distributor rotor button freeplay?

I have a MSD distributor in my 360 with missing at idle. The tach just jumps around from 1000 rpms to 1200. For a long time I just payed it no attention because at the strip or when cruising at 55 which is about 3,000 rpm theres no problem with engine miss. The other day I was making a adjustment to the advance springs and noticed that the rotor button could be moved about 10 degress or so.(without moving the advance arms). It seems there is slop, as if the distributor/oil gear drive was worn. I havent taken the distributor out and looked to see if the slot is worn yet. Just wanted to see if someone has heard of such a thing. Maybe its just in my head and something else is the matter. thanks for any advice