Check inside the distributor for any wires that flex when the vacuum advance moves. Those wires have a nasty habit of breaking the conductors inside the insulation, and interrupting the circuit under the proper vacuum and load conditions. The stock HEI ignition was notorious for flex wire problems with the pickup coil, and it's possible your aftermarket ignition system has something similar.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!