Fluctuating neg. to ground on ignition coil

Well, this is how I used to buy Rislone LOL
Vintage Shaler Rislone Engine Treatment Full 1 Qt Motor Oil Can FULL NOS | eBay

But I think you want the Engine Treatment, not the Oil Treatment which has the ZDDP. Now I do not know if the Oil Treatment will do the same inside the engine oil. I don't think I would do both....but you need the ZDDP. So maybe just do half of a small container of the Rislone Oil Treatment down the carb. Just an easy thing to do, but a long shot to do you any good.

BTW, my neighbor when I was a kid would do this about once per year on his old pickup trucks. It would clean off the valves and it would run more smoothly for a while. Us kids would love to run around in the smoke LOL