I cant believe no one even asked you this. I can just about guess that your timming chain is worn out. You havent changed your cam so you never even been in the timing cover yet. when a chain gets old you get almost 10 - 20 degrees of slack that the chain moves before the distributor even moves! replace the timing chain and gears, get a recurve kit for the weights and srings in distributor, and oh yeah... check to se if vacume advance even works, and the mechanical advance system built in distributor even moves. I dont mean the weights themselves buit the shaft itself must move. As the distributor gets old the shaft starts to rust and not let the mechanical advance built in it pivot. AND.. LOL... the place where the wights pivot.. check to see if they arent all grooved up either. You can no see that there is a lot of spots that cause timming misses. everone has missed them completely. When you do go all thru that, get a advance type of timming light to check out initial, vacume and then total advance. There are 3 different totals you must dial in. Welcome to tune up 101.