Get a diags program to check the sensors. HPDIs need CLEAN fuel. You have a host of filters, including at the non servicable high pressure pump assembly. Either check the fuel pressures at all three pumps. Low. Medium and high. The only way to check the high side is with a diags program that will read pump pressure from the sensor in the high speed pump. Do a leak down or compression check first. Check to make sure you have six sparks. Unplug the injector plugs one at a time with the engine in gear, hopefully tied to the dock or in a test tank and see if they are all working correctly. These engines will run with VST fuel pressure and then fall down at higher speeds if the high pressure pump is blocked or not functioning.