This engine uses a overboost control circuit that can be the culprit. A clear plastic line that goes from the intake manifold to the injection pump must be free of restriction. This includes the hollow bolt that connects to the intake as well as the boost control solenoid. These circuits often get gummed up with the carbon from the EGR or blow-by debris. It is also possible that the switch on the intake that controls the overboost solenoid is the problem. Bypass the boost control temporarly with a piece of hose and see if this helps.