12.7 Detroit no egr intermittent loss of engine power

Hello I have a factory reman 12.7L 60 series in a 2017 Freightliner Glider with about 300,000 mile on it. My driver is saying that the truck will run fine for a few weeks, then all of a sudden it will loose power and the fuel mileage with go from 7-8 mpg to 4-5 mpg and the truck will run like that for a few days or a few hours then come out of it, it has done this since the truck was new. The truck has also thrown a fault code 3 times in the past for injector 5, the injector was changed twice at the dealer under warranty and also had the injector harness changed out and tried a different ecm with no luck. Any ideas would be greatly appreciated