What can make my 88 s10 5 speed 4 cyc bog down an backfire on interstate in fit gear lose power on hills

I can drive it anywhere at 55 an 60 fine but when I
get on interstate an floor it to get up to 70 in spits
an sputters an wants to die an backfires till I let off
gas then I easy gas pedal down an it does it again
an it really does it on hills  I have put new
distributer  the whole thing on it new spark plugs
wires an egr an icm we set the time an it will sit all
day an run drive any where at 55 an 60 but when u
Try to get it past 60 it back fires an wants to die an
won't get past 60 till u go down a hill an it will go to
70 75 an push gas an it boggs down an try to die
an let off gas it fine but push gas it won't pick up
speed ???