Just found your post, having the same issue. 1996 350 vortec, all new delco plugs, wire, cap, rotor, intake (upper and lower) gaskets injectors and new gm trans installed by dealer. Truck has a 6in lift and a bit of driveline wind up on acceleration. same range as you described. Will only happen while keeping steady highway speeds even worse on a slight downgraded road. Did you ever figure yours out??? Thanks!
Still haven't figured it out. I also have bubbles on my transmission fluid dipstick so might be a bad torque converter. The misfire code has stayed away but problem persists. so im thinking fuel pump cutting out of a transmission issue.