Recently won a pair on ebay and started having shifting issues as soon as they were installed. This is the right shifter the one that controls RD and hence, the gears. I thought the issue might have been one related to RD but have since repladed the Ultegra with an STI 105 from other bike and sure enough, problem went away so I know it's the Ultegra now.
That being said, the problem was that the smaller lever (non brake) would often just stick. I would put pressure on it, and it wouldn't budge but often, a let up in pressure and depressing it again would then make it shift into gear. Eventually it became worse and would shift the brake lever as well or shift in the opposite direction then what it should be doing. Not fun when you are trying to climb a steep hill (this is Cyclocross build) and the bike decides to shift UP instead of down! Anyhow, I have the parts breakdown PDF (they are available on Shimano website) but would like to know where to start here.
Any pointers would be much appreciated.