1991 Toyota 4Runner Hesitates under acceleration

Hi there. I recently bought a 4 runner with a stuffed engine, I rebuilt the 3VZE V6 engine spent alot of money on various parts including suspention. The problem is very intermitent which is why im having trouble finding the problem & really need some help and is driving me mad. Sometimes when I accellerate the engine holds back unless I floor it then it clears for a while then comes back, can do it on the hwy or off traffic lights. Feels like something is breaking down, or starving for something, fuel pressure is good plenty, new plugs, leads, coil, no fault codes showing. When it clears it runs really good, responsive but when it hesitates its bad like its out of fuel but its not. Please can some one help me with this as im really stuck. Thank you