In addition to the above suggestions a cleaning and re-lubrication of the derailleur itself is probably warranted. If you decide to replace the cables and housings (highly recommended), use good quality lined housings and die-drawn stainless cables. The last loop of cable and housing going to the derailleur frequently gets water and road grime in it so don't forget that one. For good performance it is essential that the housings are cut and fitted properly and that the correct ferrules are installed.