check to make sure the cassette lockring is tight. sometimes if the cassette is loose, it could cause it to roll slightly on the freehub body and cause the chain to jump. this is not likely the solution to your problem, as most of the time it just causes sloppy shifting, but, it never hurts to check.
try all the other suggestions first. it could also be cable stretch, but that usually happens sooner, and is more of an across the whole cassette issue.
when in doubt, take it back to the shop you bought it from and have them look at it.