Shimano 6700 rear shifting adjustment issue

[kenwstr]

Hi, I have had this bike several years and done all the shifting adjustments myself without issue after sorting out some problems from the initial build. The shifting has always worked great except the last 2 rides. Before today's ride I adjusted the rear cable as per usual. That is shift from the smallest cog a full leaver through and hold. This this should shift the chain up two cogs and just rub the 3rd cog lightly while the leaver is held over. On release of the leaver, the derailleur centres to the correct position. Using this method (specified in the 6700 manual) I have had no problems. However, the last 2 rides, while shifting to bigger cogs is ok, the chain is very reluctant to go back the other way to the smaller cogs. It may sound like running between 2 cogs but actually doesn't shift and on shifting again to a smaller cog it skips 2 cogs and may still be noisy. If I adjust the cable so the shifting to smaller cogs works OK, it then won't do a single to a larger cog on a single click of the shifter. I would have to move the leaver about 1.5 clicks & hold it there until the chain shifts and then let go. The chain will then stay on the correct cog but may be noisy. So, It's like the derailleur is moving to different positions depending on whether I shift up or down.

The bike has done 13,400 km since new.

Everything seems to be lubed and moving freely and the derailleur appears to be hanging straight. I had a good careful look at that but don't have the tool to check it properly.

I checked the cable inside the shifter for broken strands, all good.

What ever could be wrong?

Ken

[Duck!]

Worn outers.