cant figure this out

New to this and have a few questions about a problen I've been having with my 99 s10 extreme, 1) the truck is a 4.3L and has 75k auto 2) the problem I've been having is the truck at low speeds 20-35 mph has no power. No matter how hard you push the gas the truck does nothing like its stalling then all of the sudden it revs up and takes off like a bat out of hell. Any answers as to why? would appreciate the help thanks.

spit and spudder, doesn't respond wiht engin rpms? HOW is it not responding is what we need