I have a 1987 isuzu pup that bogs down when i give it gas?

i'll start it up and it will run fine... for about ffive seconds. then it will start to choke and sputter kinda. i'll give it a little gas and it will rev up some then almost stall if i dont let off the gas in time. when i was driving it down the road it was doing the same thing so i basicly went 5-15 mph all the way home... what is wrong with this truck?