I have this unit in my '01 3500. When the unit is turned up to 9 it the truck shuts down at 85MPH, As the unit is lowered in number it will gradually allow you to go faster and when it reaches 0 there doesn't seem to be any problem going any speed.

 

Have you contacted TST? Did you buy it used? What gears do you have? Sounds like you hit the governor and yet you can go faster without the added power. Anyone have any ideas? Scott