My rear derailer is not shifting as well as it used to. When I shift from a larger cog to a smaller one it is late (slow). After I click in can take two or three pedal rotations before it actually shifts. It seems to happen most when going from the third largest cog to the fourth. Shifting in the opposite direction is never a problem. Is this something I can fix myselft or should I go to the bike shop? Any help appreciated.
This problem is usually a sign of excess friction on the shift cable within the cable housings. One thing you can try is to loosen the small housing found at the rear derailleur, slide it up the cable, lubricate the exposed cable with a silicone or teflon based lube, re-attach. Further you can try hosing out the shifters with WD40. If these two tricks don't cure the problem you should replace the shift cables and housings.
It's a waste of time to replace the cables without replacing the housings.