I should preface this by saying I did search, and while some come close I didn't see this particular issue:
I replaced a dura-ace 7800 11x23 cassette with another 7800 12x25 cassette of about the same wear. Prior to this change the rear shifting was flawless. Have a 2006 Spesh Tarmac, btw.
Now the problem is that when on my 53 chain ring (39-53 rings) I cannot shift to the 25 or 23T cog without the chain skipping or 'wanting' to go down a gear. All this while I have the bike on the trainer -I always test equipment changes for 2-3 days on trainer before going out, exception being tires.
To the best of my ability the derailleur hanger does not appear bent, the pulleys do line up.
I released the tension from the trainer, and it does get better, but not what I'd call acceptable, even after considering chain rub on front derailleur. I also pedaled backwards and it does not down-shift to suggest a tight link.
I've tried to loosen up the tension on the shifter cable (about 2 turns on the knob) but no help either. I suspect my chain is a little too short for this cassette but would like to hear others suggestions before I proceed.