F150/ Reckon wats wrong??

A few weeks ago my check engine lght came on and the code was 174, I was told it was my bank two O2 sensor, I replaced it and the fuel filter, the truck ran much better, until today, I was stopping at a redlight and the engine, rpms dropped to the lowest I have every saw, and it was kinda lopping, the check engine came on, after a few minutes the engine picked up and now is running good again, I checked the code again and it said 174 and 171, now the guys at auto zone tell me i NEED TO REPLACE THE o2 SENSOR, on bank one, because it is bad, they say if it is bad it can cause the bank 2 to give off false reads, whats up, somebody give me a idea, of what to do short of going to the ford dealer and giving them all my money thanks for any advise