MG Sports Cars

My wife and I were in the middle of a nice holiday week-end away at an upscale resort (I'm in Canada, eh!) in the MGB. Lots going on, parade, fireworks, big interenational bike rally. Driving slowly down the main street yesterday afternoon and the car quit, as if someone turned the key off. Mrs. Phil was not impressed.....she had to push (I helped).
My first thought was the Pertronix module installed by Jeff Schlemmer when he rebuilt the distributor. Checked for spark at the coil - lots of it. Lifted the distributor cap - the inside of the cap, rotor and the 8 electrodes were covered in what I think was carbon dust. Cleaned it up with a rag and the car fired right up.
Got me home, a 100 mile drive, without a hitch. The car only has 3500 miles on it. Anyone else have this problem?

If it is carbon, where's it coming from?...what is wearing down?....eg. any signs of rotor hitting distributor cap?....should be some evidence of the source of the dust

Graham:
The top side of the brass business end of the rotor appears to be hitting one or two of the metal contactors in the distributor cap. Lots of dust on the rotor's centre contactor and the top of the rotor where it contacts the electrode (?) in the centre of the cap. I'm going to take a closer look when I get home. On the road for work for a couple of days.