2000 Idle stumbles

I have a 2000 Cavalier with a 2.2 auto trans with air. It stumbles a little when it is idling and hesitates when you take off hard at a light. It just misses a little about every 15 seconds. I used an actron scanner and got a P1137 lean system or low voltage. I changed the O 2 sensor after the cat and that seems to have taken care of the code. It never made the check engine light turn on. It still has the stumble and it also hesitates a bit when you take off hard at a light. What all should I check now and in what order?

thank you mark