Sounds like a suction leak as this engine has no central injection pump. One of the easiest ways to find this type of leak is with shaving cream. Place the genset under all the load you can and put shaving cream on every fuel connection/filter gasket from the tank to the fuel rail, if a suction leak is present it will suck a hole in the shaving cream. Another place to look for a problem is in all the fuel check valves. The Parts book will locate theses for you. There can also be a issue with anyone of the unit injectors that will allow back bleed. I assume you have check to see if you are leaking fuel into the lube oil, if the other tests fail to to provide an answer pull a lube oil sample and have the flame test done to see if fuel in getting into the lube oil. If it is the problem is internal. Check this simple stuff out first before we get into it deeper. How many hrs on the unit and when did this issue start?
Once you know how a device, system, ect operates, finding the root cause in the event chain is easy. Until then, you will always be chasing/repairing the effects. Learning is a life time event, if you want to outsmart, whatever item you want to repair.