Not to ask a stupid question here but,are you running a vacuum or mechanical distributor and if it is a Mallory Vacuum advance are you running full or ported vacuum?I had the same problem with a Mallory on my SBC and it turned out to be a vacuum problem.Mallory distributors are set up for full vacuum,set your initial timing high and your full to normal.When its on ported every little vacuum pulse screws with your timing.Hope that helps.