1990 GMC S15

I have a 1990 GMC S15 pickup and it seems to be in very good condition. I recently had the o2 censor replaced because I had a check ingine light that kept coming on and that did the trick. But now it seems to have a surge like quality to the idle when I'm sitting in traffic it shakes like its about to die and when I take off from a red light it chokes, it doesn't die but has the feeling that it will but when I get on the Hwy at higher speed it seems to be fine and run like a top but I am now more nervous when I have to slow down. So what is your best guess as to why this is happening and what should I do to fix it.