94 Ranger runs rough on the bottom half of throttleRuns good 3/4 throttle and up

[MikeJanowicz]

When I bought my 1994 Ford Ranger it had no 02 sensors in the pipes, I put in two new sensors and it ran great for a couple of days. The check engine light came on again and it was throwing a code for the left O2 sensor. I put another O2 sensor in the left side, with no change. The truck has a manual transmission. When accelerating the Engine lacks power until the throttle pedal reaches about three quarters of the way down then the truck takes off and runs smoothly like it should. I tested the MAF sensor and the throttle position sensor both checks out OK. For some reason the left O2 sensor code keeps coming back? anybody ever have this problem or any thoughts on where to look to fix?? The truck runs better on the throttle pedal hits a certain spot not certain RPMs. If I stop and let the truck idle for a few minutes and then drive it runs good but within a mile or so it starts to skip again, sometimes it will run good for a short period of time without hesitation and then all of a sudden it will start skipping again!