Shifting problem w/ rear derailleur
I'm having a problem shifting through the cassette while I'm on the big ring. When I'm on the road, shifting is sloppy and will often jump between cogs when I stand up while climbing. It's usually worst in the middle of the cassette but it more often than not won't shift to the biggest cog at all. When it's in the stand, shifting is just fine, works how it's supposed to, and I can't detect any problems. When I'm in the small ring, shifting is just fine. It's only the big ring where I am having trouble.
I'll increase my cable tension while on the road and that'll help shifting in the bigger cogs, but the lower cogs will then go out of whack. It'll also have an adverse effect on shifting while in the small ring.
Any ideas where to look next? I can't figure out what I'm doing wrong.