Hi, I have a shimano 105 9 speed shifter and 9 speed derailler.The shifting seems fine and I have fiddled with it a lot and done a lot of tweaking. It works fine under pressure on the upper two cogs and the lower two cogs, however, when I stand up with the chain on any of the middle cogs, it jumps. This is obviously very frustrating during a cyclocross race and ruins my momentum/mindset. When I pedal sitting down, it doesn't skip. I am going to change the chain and put a new ultegra derailleur on, I hope this will fix it, but any other opinions would be appreciated.
It shifts fine on the workstand, so it's very hard to see what the problem actually is.I have adjusted the cables for stretch etc. and taken that into account.