I would second Oak's advice, but would like to ask another couple of questions. You mentioned that you changed the cables, did you replace the housing as well? Often times the housing that feeds into the rear derailleur will show these same problems. I also might wonder if you looked at the dropout; I have to assume that it is straight? Lastly, how old is your rear derailleur? Try this to answer that question: have your chain in the smallest cog on your rear wheel, take the derailleur and push it so that it is now on the lowest/biggest cog and let the derailleur go (all without using the shifters). If your derailleur pops back to the original cog, then the spring tension is fine... if not, then the derailleur needs to be replaced.