I recently picked up new 8-spd Record hubs, new (used - 1996) Record 8 Ergo levers (right was rebuilt ~300 miles ago per the seller), a new 12-23 Ultradrive 8 speed cassette, a new Wipperman Connex 8 chain, and new Torelli cables/housing. Remaining components are ~1994 Chorus crank w/ original 39/53 c-rings, ~1994 Chorus front derailleur, and ~1997 Chorus short cage rear.
I have installed, adjusted, and lubed everythng correctly (as far as I can tell), and both derailleurs shift fine when the bike is in the stand. But when I put pressure on the pedals, the rear mech's cage starts pulling forward on its hinge, dramatically slackening the chain, and then jumps back. The more pressure on the pedals, the worse this "bucking" behavior. I can still shift when pedalling, and furious back-pedalling (a fairly reliable indicator of a missed shift or misadjusted rear mech) is perfectly smooth, regardless of which cog the chain is on. I was able to reduce the problem by swapping out the new chain for the old Sachs one, which still passes both the 75% and 100% Chain Checker tests, but is slightly stretched compared to the new one.
I am at a loss as to the source of my problem. Chain length is correct according to the "rear mech pulleys are vertically aligned when the bike is in the tallest (53x12) gear" rule of thumb. And what's very weird is that the cage is taking up slack, not absorbing the pressure of my pedalling - the cassette is doing that. The derailleur cage is being pulled forward, against the movement of the chain, which is moving backward through the pulleys!?! Could there be a compatibility issue between the Ultradrive cassette and either chain? Or is there some break-in period for new cassettes and/or chains? Could this be an alignment issue? Any other ideas?
Thanks in advance,
Steve