Sounds Like Fuel
Some machines are more tempormental than others with fuel. I would drain the fuel tank. I know this is a pain but there might be debris clogging the bottom of the tank. I have had machines that suck in debris and clog the line. I use an air line and blow back into the tank. When the motor can't get its required fuel flow when under load, it will bog down. Once you let up on the load, the motor will come right back up to high RPM. You can run a machine wide open sitting in one place with no hydraulic function. Once you move the controls, the machine will bog hard. This is a classic fuel problem.
The main problem I have seen over the years is the fuel supplier. Some are better than others. You will not believe the junk you will find in your fuel if you have auto delivery from a fuel truck. Some machines will eat it up, others are very delicate. I know for a fact the Cummins is a delicate fuel machine from hard experience with multiple motors in the field.
**You also have to watch the low pressure fuel pump which is the same unit you hand pump to prime the machine. This is key to performance. When it gets weak, you will have a hard time priming the machine and keeping it running. Other than these two problem areas, the Cummins is an extremely reliable machine. Although not excatly a fuel "sipper".