Bucking and lack of power " But not all the time "

I have a 1995 chevy S10 and for some reason sometimes while accelerating
especially uphill it will start bucking like a bronco and will lose power . If I pull
over and turn the truck off for a few minutes and then crank it back
sometimes the problem will vanish but not always . It seems to happen on
longer trips on the highway and not so much driving around town . I'm pretty
sure that it's a fuel system issue . I've replaced the fuel pump & filter and
everything that I can think of between there and the throttle body but maybe
not everything . I've also replaced spark plugs , wires , distributor cap ,coil
and a
few other ignition system parts that I thought might be the problem but I think
it's a fuel system problem because of the fact that it runs fine it just loses
power going up hills sometimes . Any Ideas ??