1994 GMC C1500 slight engine miss

This vehicle started to "bumble" slightly the other day when towing my boat. The last time I had this it was much worse and turned out to be the dist. Cap. That was about 18 mo's ago. I opened the cap up and it looks good. What concerns me is there is a ring just under the rotor, it is about 6 inches around and about 3/8 inch thick. It is loose, just sits there and can be lifted up and down slightly and turned about a 1/2 inch or so in either direction. Is this part acting normal or is it broken? If broke does the dist. Have to be replaced? Thanks, dennis