Originally Posted by
vasuvius
+1
check the derailleur alignment.
I recently replaced the cassette and chain (10s Ultegra) and had a chain skip on exactly 1 gear (small chain ring and 17tooth cog). Turned out to be derailleur alignment.
I had a brand new frame with the hanger bent 1 or 2 mm, couldn't even see it with the naked eye. Just that little bit throws off the whole tune. You can check it if you have a spare wheel with a threaded axle end, it screws right into the hole for the rd then you measure the distance between the two rims at the top and bottom. Or a shop can check it in about one minute.