Engine surge?

My Sonoma's engine will rev a couple times when slowing down. It usually happens when it slows to about 20 mpg or less. Sometimes when I drive slow then put it in park it will idle faster than normal. But if I shut it off then restart it, it will idle normal. The check engine light just came on today but it's been doing this for a while. It seems to shift fine when accellorating, the trans doesn't slip. I've had trans service done, a tune up & a new fuel filter & my gas mileage still fluxuates up & down. Does this sound like an O2 sensor or throttle sensor? I've put way too much money into many repairs on a weekly basis. Can you help me?