Fuel Air Ratio too Lean and Not Downshifting

97 Ford T-Bird, 130,000 miles, 4.6L V8:
My check engine light has been on for some time. I have had it checked and my oxgen sensors are reporting "Too Lean". I replaced all the oxygen sensors and cleared the codes. I drove it for about 3 days and the check engine light came back on. I had the codes read and they were reading too lean again. When I push the gas pedal to the floor it just revs up without downshifting. Do I have some kind of air intake leak? Please help!