I recently upgraded my circa-96 Giant TCR (105 groupset) from its original 8-speed 12-25 cluster/chain to 9-speed 12-23 cluster/chain. Shifting on the 9-speed cluster is perfect, except when the rear derailleur gets to the second to the smallest cog (13?). It stays on it briefly but jumps off onto the 12. I've fiddled with the cable adjusters on the downtube, as well as on the rd. Can this still be fine-tuned? Or is my STI shifter not compatible with the 9-speed cluster?