I6 surging/struggling under acceleration

[Anorak]

So I have a 1983 Chevy square body pickup, 3 speed with a granny gear, with a I6 in it, recently I started to notice that when I was accelerating in 2nd gear it would start to surge/struggle, but when I let off of the gas for a second it would be fine, and i could still accelerate passed that. That led me to think it was the fuel pump on the engine going out, because it seemed like it was starving for fuel, but I just put a new one in and it didn't change anything.

So I'm out of ideas of what it might be, so I was curious to see what you guys thought.

Thanks