The switch that controls the high idle plunger also internally advances your injection pump. That is the rear of the two wires on your pump. It sounds like the switch is working properly. Mine shuts off when the coolant reaches 130* The plunger can be adjusted by threading it in or out with a 1/4" wrench. Until the internal advance on your pump shuts off the truck will behave like you describe. I cruise along and don't push the truck hard until the advance shuts off. You should be able to hear the difference (less diesel knock when off).