What could be causing a gray metallic carbon type buildup in my old style points distributor?

I can clean it out, put a new cap and rotor on, then after a small amount of running, this buildup is back and causes hesitation and hard starting.

The points look ok-I assume the dist shaft or maybe a bearing is coming apart. This is on an older 4cyl 181 c.i. chevy engine.

Do I just need to get a rebuilt dist?

What does the center post look like inside the cap?

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Leo Paugh
Maryland Chevelle Club #017
A.C.E.S.#3731
progress has little to do with speed, but lots to do with direction.
Maryland Chevelle Club (http://www.chevelles.net/mcc/mcc.htm)