Think again! I was faced with this same problem last night. Spent quite a bit of time tweaking the adjustment of both derailleurs and the problem still wouldn't go away. Eventually traced it down to the fact that one of the links in the chain was slightly, and I do mean slightly, twisted. Whenever this chain came across the rear cassette, it had a tendency to ride up to the next gear and pull the 2-3 links behind it in that direction as well, which lead to a momentary skip. I found that the problem was worst when back-pedaling; the chain had a tendency to completely jam up at the rear cassette, which I didn't see when pedaling forward. To find the problem I ran the drivetrain forwards and backwards by hand and noted the position of the master link when I got the skip. Once I isolated the problematic part of the chain I watched the links as they went across the rear cog and noticed that the teeth weren't centered between the side plates. From there, I was eventually able to spot the twisted link.