Re: sputtering feeling at lower rpm's around 50-60mph....
If you just bought it and don't know the maintenance history, IMO, you should just do a complete major service/inspection. Include all fluids/oils/filters, antifreeze change, spark plug, spark plug wires, EGR Valve, MAP Sensor, PCV Valve (also clean behind the throttle body butterfly and buy a catch can), and if that doesn't significantly improve the mileage, also change all the O2 Sensors. That's what I did with mine when I bought it with 48K on it. I didn't have a bad mileage problem though, so didn't do the whole enchilada until 60K. I changed the EGR Valve at 54K (failed) and did the O2's a little after the 60K service, because one was failing and one was getting "lazy". A major service will run you about $500 in parts (fluids/oils/filters, etc.) if you do it all yourself. If you take it somewhere else, make sure you watch them like a hawk. Many dealers (including mine) don't even know how to service them properly.
05 LX/RT & 06 WK/GCO, modded similarly, MDS "ON" Light, Vortex Gens, Debadged, DashHawk, BT Catch Can, SRT CAI W/Add Air Inlet, K&N Filter, Sonnax Trans Press Booster, SC Tuner W/Adv Trans Tuning App, DS Tuner, Cust tube headers w/3 in. catback, Bilsteins, SRT frt/rear bumpers, Eibach SRT Lwr Sprgs & Sways, FRI Sidewinder Cam Adv 2*, 6.1 valvetrain, SLP 25% UD Pulley, BBK 90mm TB, SOS Coils/.060 plug gaps. 20 in. SRT Reps.