What should I do if my car keeps Idling back and forth?

What is your question?

My car started idling back and forth yesterday afternoon I was getting ready to trade her in for a newer car. It would only do it at red lights and while in park and in nuetral. A guy told me to put a gas treatment in it (seafom) because I told him that no new light came on just the same one and that light was on because i have a whole in one of my hoses. Could someone please help me.