I have an 08 VT 880 with a D16. I've been having electrical issues.

CHECK YOUR GROUNDS,

 

I've had the maxi fuse from the batteries cause something similar on my D16. The voltage on the dash would fluctuate between 1-2 volts lower than what the alternator was putting out (14.2V). Eventually everything (headlights, interior lights, dash, all power) would shut off when hitting bumps in the road, sometimes it would come back almost instantly and other times I would clean up the grounds and cables and it would make its connection and start up again until the next bump in the road. As a temporary fix, I bypassed the fuse and ran direct to the battery until I could get another one installed.

If you are not losing all power to the truck, only certain components, then you might need to grab a meter and a wiring diagram and start testing your wires. If all of the wires check out, ECM would be the next step I would pursue. You might be able to get better help from someone like @nasriza . D16 parts can often be hard to come by, so hopefully it's something simple.

https://class8truckparts.com/media/file/vn-vhd-us07-schmatics-volvo.pdf

 

Join the facebook group Volvo Truck Masters, they have many knowledgeable Volvo mechanics on there, some who even own their own shops that can better assist you.