Mrkrad

I have a low boost scenario; intermittent. no codes at all.

All maintenance on my car done religiously.

Only thing suspect is several year old non-attended forge diverter.

Stage 1+ requests 2500mbar; several days in last week felt like 7psi.

then suddenly back to normal (same driving time no key out).

No codes; no underboost; nada.

ianbis

was an intermitent fault code for the N75 valve

diagnosticator1

Even though it felt like boost was only 7 psig, what was the actual boost achievement when torque was below expected? The first thing that I think of with the intermittent symptom, is a sticky mechanical item like the wastegate, or as you mentioned, the Forge DV. And, relative to the stock boost range, 7 psig may not be low enough to set a low boost DTC. Your concern that the DV needs cleaning/greasing is probably involved.
If the symptom reoccurs after servicing the DV,check the wastegate for smooth stroke without sticking partially open. Some DTCs require the fault to exist beyond a certain time period before the relevant code will be stored.