ses code, lean banks

[mudboy]

I got a 98 chevy 3/4 ton 4x4 has the 350 vortec engine in it. it has a miss to it when trying to hold the rpm's, like when the cruise is on or just maintaining a steady speed. I have had it into a dealer and said it need a tune up and injector flush, put in new plugs, wires, cap, and rotor and have run 2 fuel tanks of seafoam, 1 tank of lucas injector cleaner and 1 tank of stp injector cleaner through it and I run 91 octane gas. still has a miss and still has the ses light(have cleared it twice) with a lean left and right bank code on it 171 and 173 i think they were. from what i have found it can be anywhere from mass air sensor, timing, fuel pressure(is good), O2 sensors but can't get any specific answer on which or what it might be.... anyone have or heard of this problem???