My guess is that the AIR system is not shutting off when it is supposed to. That would cause some of the air that is supposed to go to the cylinders to go to the exhaust instead. Since it is mechanical, the higher the rpms, the more air gets taken from the cylinders. So you end up with too much gas going into the cylinders and the O2 sensors may be getting confused because of all the unburned air getting put into the exhaust. All the extra (unburned) air in the exhaust may also be cooling the sensors enough to confuse them as well.
I forget to tell you guys I don't have smog pump , I and I don't have those two selenoids and pipe from heads are closed or plugged ,
I have Hypertech chip and I try take that out but nothing.
Yesterday I bought two new O2 sensors and installed them but same thing
here is my mods if you guys don't remember them
Flowmaster American Thunder Exhaust Systems ,1 5/8 Mac Long Tube Headers , Mac O/R H-Pipe , King Cobra Clutch , FRPP 3.73s gears ,Ported/Polished GT-40 Cobra intake , Kirban Billet Adjustable fuel presure regulator Hypertech chip , Walbro 255lph HP , MaxSpeed Springs 2" lowered , Home Made Short Shifter , MSD 6A Ignition , Blaster High Vibration Coil , BBK cold air , Short belt w/out smog Pump and A/C, Timing 14* , 180* Thermostat , Manual High Speed Fan Switch