Gear skipping problem
 

I have a Ultegra10-equipped road bike with a triple chainring and a 12-25 cassette.

I am having problems with shifting the RD; it skips the 9th largest cog, both down shifting or up shifting. It'll stay in the largest cog, but when I go to a higher gear (smaller cog), it skips the 9th cog and goes to the 8th cog. Then it'll skip between the 8th and the 9th cogs. Very annoying.

Any suggestions?

thanks,

The Large rear cogs work with the Small front ring.
Are you in the Small ring?

Yep. the small chainring isn't rubbing the FD

Friction Shifters fix everything.

Don't try that cross stretch of the chain. You're at the limits. Just shift to your middle ring.

I'm not cross chaining. I'm in the small chainring and I'm trying to use the big cogs.

Before anything else I'd check that the gear hanger is not bent. Following this I'd check out the rear section of outer cable ending in the RD as it can suffer from corrosion causing the change from low to high cogs to hang up.