Once upon a time, in a far away place( with $.30 gallon gas) I did a lot of tune ups on various cars----it was quite common to get a car with a GM dist that had re occurring pinging or sluggishness---it generally came dowm to a seized or partially seized upper shaft in the dist. It would keep the dist from advancing, the timing would get set up and then it would advance and pinging would start---then timing would get set back and eventually the dist would retard and stick again causing lazy/sluggish---

If you have a stock type GM dist, I suggest removing it, taking off the lower gear, removing the shaft and the the advance mech and polish/free up/lube the shaft making sure it is free as can be.

Another area of concern with vacume dist is the proper type vacume unit hooked to the proper source of vacume---ported or timed---

If all else fails get an msd retard thingie and when it pings just dial it back a few degrees--at least you will find out what it wants for timing and you can recurve the dist to suit the fuel