87pickup

I bought a 87 pickup stock that had been sitting for a few years!! I put the basic parts in it to get it running! And changed the suspension to 6" springs and 35" tires and 4.56 gears& lockers, some minor inter parts little by little! It has been running great until now I cant seem to figure out what is wrong? It starts fine and is in time, but as i run at a higher RPM such as highway speeds for a period of time!!! It starts to have a hesitation when going from 0mph-and up!!! i changed the plug wires and plugs it helped!! New distributor and cap, new carburetor, new fuel pump!! i don't know what is wrong! Can someone point me in the right ?????