idle fluxuates, 1200rmp and drops to fluxuating idle at stop

It started with the check engine light. Took car to auto zone coded 171 changed mass air flow sensor. Nothing. Changed 1st exhaust sensor light went off a. thought ok took to work light came back on. Engine idle going up and down around idle rpm. Would go to stop sign engine rpm's 1200, then drop and fluxuate around idle stalled once. Put seafoam engine cleaner in tank, helped a little idle still bounces but sometimes runs ok then rough then starts to fluxuate again.

Update from Oct 7, 2016: Took the car to Chevy dealer today service department called back and found a vacuum leak at the air intake at the manifold. also found out nut was stripped where leak was this engine was put in by a locale mechanic. won't be going back to that mech ($1800 for engine with 99,000 miles) $220 so far. Wish that is total will be back later after I get ok an total