Lately I've been having a problem with my shifting. It seems that when I shift down on the rear der. that I usually have to either keep my thumb pressing the shifter a little longer or I have to nudge it a little to get it to get in or stay in gear. Also, sometimes after a time of being in gear it will just jump out and the chain will start skipping, and if I pedal a few times it will jump back in or I have to nudge once again with my thumb.
This only happens going down gears, never up. It also appears that it only happens around the 7th, 6th, 5, and maybe 4th gears. I have an 8spd. Thanks for any advice on how to cure this problem.