Seems like everybody has hit on the "usual suspects" but your comment about "metallic dust" makes me wonder if you might have/had a dry lube pot on the side of the distributor. Giving it a turn with each lube job is often overlooked. I'd give the distributor/oil pump shaft a side-to-side wiggle and see if you don't have slop between it and the distributor housing.
A worn distributor shaft can have your rotor gouging out metal from the plug terminals......might have created enough metallic dust or a hairline crack in the upper part of the cap to cause a crossfire. Also effects the dwell/points setting. If everything is snug and your lube pot is pushing a little lube onto the distributor shaft, I'd take Denny's advice and pop in a new coil.
Dave
