Originally Posted by
nonfortuna
This sounds to me like you are fighting against the high limit screw. The OP states that the issue arises when shifting from 2nd to 3rd, nothing about the 1st to 2nd shift. Try backing out the screw labeled "H" 1/4 turn or so.
This is a really good point. Look at the derailleur and see if you are up against the limit. If so, back it off slightly. Being up tight against the limit would give the symptoms you are seeing.