For the last few weeks, my right shifter (shimano ultegra) has been getting stuck when shifting from a 15 tooth gear to a lower gear. I have been spraying WD-40 and it works for maybe one ride. I also tighted the cassette and cleaned it, along with the chain. So my only conclusion at this point is that my shifter is toast. Should I start looking for a replacement or is there still hope. I estimate this shifter has seen aproximately 15k-17k miles.