2000 Chevy Blazer At above 3000 RPMs I have no power

Hello, I have a 2000 Chevy Blazer. When I am driving at normal acceleration the Blazer runs okay. But when it is held to the floor (hard acceleration) it has a problem. Once the rpms reach 3000 rpms it has absolutely no more power. But, if you somewhat ease into the throttle from 2,000 rpms to around 4,500 rpms it will sometimes smoothly accelerate properly. But not always. You can put it in park and put the pedal to the floor and it spits and sputters at 4,000rpms and above. My mechanic suggested that the catalytic converter was stopped up. Where I live there is no emmision laws. And this is somewhat of a farm vehicle. So, he said it was okay to cut the converter off. So, I did. This unfortunately did not solve the problem. None whatsoever. Shortly after removing the converter I took it to my home town muffler shop and they welded a new pipe in the place of the converter. Shortly before this the Service Engine Soon light came on. I took it to AutoZone and they ran a diagnosis on it and said that the 3rd o2 sensor was bad, the one after the converter. What can I do to fix this? I have replaced the plugs, fuel filter, and oil. But none of these have solved my problem. Please help! Thank you soooo much.