This may sound a bit ridiculous and it may just have been a coincidence. However my friend has this problem and i found that he had pretty much just over tightened every screw/bolt on the shifters. Loosened them slightly and things seemed to ease a little. As i said, couldve just been a fluke.
new cables is also a good shout as dirt grime etc inside the outters would cause stickyness 🙂