Diode pack/ regulator on back of alternator failing, it’s bound to have a smart alternator that disconnects when accelerating etc
You should probably get it on diagnostics for live test whilst running
Removing and flushing your existing intercooler with petrol/ solvent/ diesel can also make a big difference, they do clog and suffer restriction after a. few years, just that process worked wonders on a 200 tdi I ran a few years back