Performance problem with 94 GT 5.0

[bflynn]

My 94 GT 5.0 starting having a problem. In low gears it is the most noticeable. It feels like I am pulling and lead anchor, it just won't go. I am getting about 50 miles per tank less on fuel milage. Top end seems to still be there. If I kick it into passing gear, it just ups the rpm's and does not acclerate. I have had it tuned up, replaced the catalytic converters, the oxygens sensors. It is better but I can still be beat at low end by hondas. Just prior to replacing the 2 O2 sensors, I was receiving the following codes. Any ideas what my problem is?

172 Oxygen sensor not switching - system is or was lean - Single, Right or Rear HO2S - Fuel control
176 Oxygen sensor not switching - system is or was lean Left or Front HO2S - Fuel control
181 Fuel system was lean at part throttle Single, Right or Rear HO2S - Fuel control
189 Fuel system was lean at part throttle - Left or Front HO2S - Fuel control