2007 Specialized Roubaix 53/42/30 Shimano Ultegra chainwheels 12-25 Shimano 9 speed cassette SRAM chain - entire driveline has about 1000 miles on it and was all replaced at the same time.

The bike has about 8000 miles on the frame.

Ultegra RD6500 rear derailleur - The problem started about 300 miles ago when I replaced the rear derailleur cable and housing.

I've been doing my own maintenance for 40 years and many bikes but can't seem to figure out this problem

Rear gears shift smoothly and effortlessly - everything works perfectly while spinning

When climbing (only climbing) under load in either the 23 or 25 tooth cog the chain wants to skip back and forth between the two. It does not matter which gear I start in the chain partially shifts to the other gear then skips back.

The chain is not skipping on the selected cog - I can see it moving from cog to cog.

If I spin while climbing the problem does not occur - only if I put more than several hundred watts into the pedals does the skipping occur.

The problem occurs with any chain wheel.

The problem does not seem to occur if I stand and stomp on level ground but I can't do that for very long in such a low gear.

I have fiddled endlessly with the rear derailleur but can't find the correct adjustment.



What am I doing wrong?