If it's not the O2 sensors, and if you're over 30k, change the fuel filter and spark plugs.
My car stuttered during acceleration really bad and my mileage tanked like you describe.
Did the above (with oil change) and she's been running perfect since. Don't underestimate what a fuel filter change can do. It took all but 20 min to do my 1st time. Super simple; remove plastic cover; 2 clips; pull out the old one; clip in the new one in correct direction and the filter change is done.
But if you see codes for O2 sensors, then bingo. There's the culprit. Regardless, the fuel filter oughta be on your list cause if you let it get clogged up like I did, you will run into similar problems again.
2007 Silver V6 Auto 3345#
K&N CAI; Bama 91Race; ASP UDP; Flowmaster AT; FRPP TLock/410s; ScreaminDemonCoil; JBA PowerCables; Deka ETX14Battery; Mishimoto PerfRad; Steeda RadSupport; BMR KMem/AArms; JBA Shorties; Moroso Tanks; UPR LCAs
185# Wt Reduction 3160#
120# FRONT=EngineBraces 5#; Rad/Support 15#; Firewall/Fend/Hood Mat 5#; Intake 10#; UDP 5#; Battery/brace 25#; KMem/Arms 35#; Headers 15#; Tanks 5#
65# REAR=SpareTire 30#; TireJack/TBar/Trunk Lining/Floor/Molding 10#; Axle Wts 10#; LCAs 15#