Hello. I have a pick up truck 2001 Chevy Silverado 2500 2W. When I am on the road and raised the speed, only reached 70. If I push the pedal to get more speed, the engine then lower the speed and I have to go to the side of the road to put it on parking and then continuing. The truck never turned off the engine while running, its only that only reach 70 and then when I want more its go down. I don't know if the computer or transmission sensor or what is. Before I go to the mechanic if you can give give me some advises would be great.