engine idles up and down

when crank my 1989 gmc 1500 350 engine up  it idles up and down like maybe the
timing is off but not so.  When i take off it suptters but do does backfire.   IT acts like
it is starving for fuel.  When i finally get to a stop sign, I have to hold my foot on the
brake and rev the engine way up to keep it from shutting off.   Going up a hill is a
tough issue.   Now when i finally get it out on the interstate and get down in it hard.  It
will sputter and act like it is going to shut but then it catches up and runs smoothly for
miles just as long as i dont slow down and try to speed back up again and then it does
the same sputtering lurking and jerking.   I have replaced the distributer, rotar cap
wires plugs and throttle body gasket.  Still does the same thing.   I had the fuel
pressure checked and the mechanic at the garage said it was running a little below
normal but not bad for the age of the truck. I did replace throttle body sensor one time
but the truck would not run but shut off.  Its ok.   It is driving me up the wall   Help!.