pewter02z28gs

i keep getting code p0134- insuffcient circuit activity bank 1 sensor 1. i clear it and then 30 min. or so later it comes right back, the car also struggles at 3500 rpm at wot but then comes back to life at 3700. i think i should also mention that i have long tubes going into slp loudmouth 2s and then into side pipes,it sounds good but doesnt seem to run right probably not enough back pressure just my quess, also my gas mileage its not good which im pretty sure is a result of the bad o2, anyone qot any ideas on the cause or should just replace the o2. the o2 is a bosh o2 for 2002 vette.

2001NBMZ28

If you don't have a scan tool to see what it's doing you could swap them side to side, clear all codes and see if it throws P0154 for the passenger side...