it sounds like your cassette is worn out, it might look fine, buts its probably worn and only skips on your most popular gears, putting a new cassette on with a old chain can cause the cassette to easily wear out in a two month time frame (as i know from experiance), just putting 15miles a week on a old chain, new cassette setup is all it would take. most people dont know thier cassette is worn out untill they put on a new chainring and thier chain skips on the cassette just like in your situation.