Originally Posted by
Iride01
Make certain it's the STI and not the cable binding somewhere. I might pull the far end of the cable out of the housing till getting to the one that is under the bar tape. And then put a little tension on the cable as I shift up and down. And if that's no change, I'd remove the cable entirely and try to gauge how much resistance as I remove it through the STI. It's possible you might have a frayed cable in the shifter. Those usually show other issues long before being thought of as a gunked up STI.
I wouldn't flush a STI unless it's a last ditch effort to make it work easily.
Have you clamped anything around your bar that might be collapsing the cable slightly? I did that once.
I will double check, but I am pretty sure that the gunk is the problem. I had the exact same behavior in the exact same shifter one year ago, and it was fixed by flushing. That was done for me (quickly, and maybe not very thoroughly) by a mechanic at an aid station during a century ride ten months ago. As RCMoeur says, this may not be the ultimate solution, but if it works I am OK to do it every now and then. I don't have access to an ultrasonic cleaner anyway....