I have 6600 shifters on my bike with 20,000 miles.

The rear der is shifting erratically. Have replaced the shifter cable, and chain. It has the problem with multiple wheels with different cassettes. Have tried cable tension adj. and limit adj. I have cleaned out the shifter with WD40 followed by good lube.

The problem appears to only appear on shifting to larger cogs. When in the 12 cog, shifting to the 13 fails entirely, then it is erratic going to bigger cogs from there, probably from the bad start.

I MARKED THE CABLE BY THE REAR DER AND I CAN SEE ON THE FIRST SHIFT FROM THE 12 COG TO THE 13 THE CABLE MOVES A VERY SMALL AMOUNT, NOT NEARLY AS MUCH ON OTHER SHIFTS. And of course no shift occurs

My only thought is that the shifter is worn out? Is it possible for the first shift to be bad, and the rest OK?

Thanks in advance for any assistance.