66 Dart still surgeing.Electrical??

I really doubt a bad rubber ring in the balancer is causing the timing to jump back and forth. If it was that loose it'd surely come apart. Timing jumping around is usually a worn bronze bushing in the block that the oil pump shaft goes through. That's the same shaft that drives the distributor and when it gets worn it slops around and causes erratic timing.

BTW: The balancer Johnny showed is the old style 74 or 75 on back. Yours should have the new style which you can see the rubber from the front.