The usual case for a bad FICM is that the output fall below 48 volts DC when cranking or running or both. Inside the FICM is a DC to DC converter that changes your battery volts from 12 to 48 to drive the hydraulic spool valves on top of the injectors. So what has effectively been done is to mount a precision piece of electronic equipment on top of a diesel engine so it can get all kinds of thermal cycling and get the dog crap shaken out of it. The main reason for this is to keep the wires to the injectors short so there is less voltage drop and other factors that have to do with timing and harmonic generation.
Now on to the meat of the subject if the Capacitors in the FICM are flaking out I could see “maybe” less performance when it is hot. The usual failure mode is that the solder joints that are made with lead free solder ( thanks to the EU ) crack and your performance suffers until the thing warms up and the joints expand back together and then you get full voltage. Without knowing the FICM output voltages when cranking and running hot and cold it is very hard to say what your problem is. Your problem also sounds like a dirty EGR valve. What mods have been done to this truck? Any tuning? What is the repair history?
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FICM repair/upgrade
Fix UR FICM!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!