An experience I recently had, with regard to HEI distributor use, in my 69 Nova with big block, was having to cut the original resistance wire out of my original harness and using a larger guage wire to handle the "draw". My old resistance wire was causing the same problem as you desribed. No problems afterward, and the engine seems to be running better and the cam seems to be "hitting" harder. I guess the spark has been strenthened.