I had the exact issue with my small block. Crank trigger isolated the distributor from the problem. I'm sure its the distributor. I used to think it was electrohysteristis or something with the pick up. I was just thinking today that if the oil shaft were to lift it would twist as its lifting being the gear is helical cut and hence moving the timing. I some how need to hold the shaft down as well as tighten up the distributor shaft wouldn't hurt.