Why is my 4.3 misfiring at 60mph+?

Im pulling hair out because of this. I have a 98 s10
with a 4.3. Everytime im driving on the highway, at
about 60+ itll start missing or when im in the
throttle for a while or when im going up a hill. If i
throw it in netural and shut it off for about 5
seconds and turn it back on, its fine or if i back out
of the throttle for a bit itll go back to normal.
The couple before me had 700 bucks worth of
ignition work done to it a couple months before i
bought it and i even changed the fuel pump and
plugs and its still doing this shit. Help me before i
ghetto shoot this fuckin 4.3 and send it to kingdom
come.