over fueling, stalling, backfiring
 

so I have a 94 and for the past 4 months its been over fueling back firing and stalling. I have changed the o2 sensors about 4 times now due to napa selling me the wrong ones (down steam sensors instead of the up stream sensors I needed) I have also changed out the mas air cleaned the throttle body numerous times, ive changed the throttle position sensor. there are no vacuum leaks, spark plugs and wires have been changed, new battery. does anyone have any ideas I could really use some help.