I have a 1998 GMC Sierra, 2500, 5.7 engine. the engine and transmission rebuilt, when I am carrying a load or at higher speed. I would accelerate and the RPM goes up to 3500 for a second and then goes back to 2500 and keeps going up and down until I let off the gas and then graduate press down. This will happen when I have a trailer at lower speed but then I have seen it when I am traveling highway speeds about 70 and 80 and going up a hill. I have had the computer reprogram, new air filter K&N, checked the air master controller, looks clean. Could use the help to fix this problem. Thank you
