The cruse control is part of a closed loop control system consisting of the ECU and the injection pump. The ECU monitors the motor speed via the speed sensor. If the engine RPM changes, the ECU will direct the injection pump to increase or decrease the amount of fuel injected into the cylinders. A secondary control function is to control the angle on the turbo inlet vanes that controls the amount of boost needed.
Most likely points of failure are the speed sensor, the N-75 valve that controls the turbo, and the injection pump.
A good diesel pump shop that has the VE certification can check out the pump. They might also be able to check out the other items as well. Otherwise drive very quickly to a good repair shop.
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