it should but not always.. the surging idle could be almost anything from vac leak to bad fuel (or pump) and all sorts of things in between. there is a few sensors in there too... run ecu diagnostics and see if that doesn't flush something out..
i really doubt it has anything to do with the belt... but change it you must.. t-belts don't really streach like the old chains do, they just tend to snap but never know.. even if it jumped a tooth or two the timing would be messed up but not really the idle..