1991 Other Isuzu Models

I have an Isuzu regular pickup 1991 4 cylinder 2.3
The problem I was having was that I was driving, the speed would suddenly go down and I had to pull over the roadway for a minute because even if I pushed the acclelerator it would not pick speed, but ithe engine did not go off, or as I pressed the clutch for a while it recovered again for a while and goes back to the same problem again.
A tune up was performed, timing belt replaced. It kept doing it. Recently the oxygen sensor was replaced because its lamp on ther dashboard was on, but the light is istill on. The carburator was cleaned and the fuel pump replaced. I thought it had solved the problem because the first day after the job was done the car worked well, but the problem came back, this time with the engine going off. I am able to keep it running because since it is a manual transmission, I just press the clutch, bring it to a lower gear, release the clutch and it starts again.