New Engine, Same Problem (400 Cummins)

That's it. I sat behind or above many BC Cummins, and air in the lines or filter will do that.

 

As pup stated I would disconnect the wire and drive like that and see if there is any difference. Could be air as 201 and Oxbow state. Possibly pick up tubes as you replaced all the lines. You would need a clear line and fittings and install that though sometimes there will be small bubbles regardless if there are problems or not.
Apex Engine Governors: Apexinds.com
There is a $48 rebuild kit but that does not include the solenoid diaphragm that is included with the $199 solenoid.

TP-1050 rebuild kit for both the Teleflex & Apex TR-1001 & TR-2001. Contains everything needed to rebuild the main governor body. Kit consists of two custom made springs, two sealing washers, filter screen, special 10 micron filter, dynapore disk (TR-1001) O-Ring (TR-2001). Special care needs to be taken when rebuilding the governor to make sure that the dampening washer is clean and the center hole is open and unobstructed. This is located under the 10 micron screen on the lower inlet on the front of the governor. We recommend that you replace your fuel lines when you rebuild the governor.

 

I was just wondering about the lines. I’ve experienced an older return line breaking down on the inside causing rpm fluctuations. I’ve also experienced a bad hose on the suction side sucking air and causing the rpm to take forever at times to return to idle….

 

Moisture can do that too. You suck up water, it takes forever to get rid of it.