[Monkey Wrench]

just a "maybe" but you could check that the screen in the tank isn't blocked, causing fuel starvation. there are also little filters in each injector , perhaps those may cause other issues if they plug. the injector parts are cheap or there are places that can clean and bench test them for you, examine the spray patterns. some people exchange or send out or replace or clean them by themselves.

if you are extremely cautious and careful of the fuel and fire issues.. do it outside and with a fire extinguisher rated for a fuel fire, and a cold engine and all the other logical precautions...

you can pull the injectors and entire fuel rail to observe how well they shut off with the key turned off . If they don't seal and continue to empty the fuel rail after key off, then that can cause the pressure to not maintain. if that happens you may also see white smoke on startup or maybe one carboned up plug.

on mine and likely yours there is a thing near the fuel pump that contains the check valve , you can probably replace it for not much but it's probably just a spring and ball so cleaning it may be enough. bit of dirt may be preventing the ball from sealing against it's seat. there are some differences in the fuel system that depend upon year.

using a fuel pressure gauge makes sense, if the in tank fuel screen is blocked , pressure would drop.

could also be that the engine shakes or twists or moves more at higher revs and maybe that affects cabling to engine sensors.

in my volvo I had an issue where a injector was not closing, it was emptying the pressure from the fuel rail into cylinder 3 upon every shutdown. I was confused until I observed it. a mechanic may have found that by other means that do not require spraying fuel around and the dangers of doing that. Perhaps by swapping injectors. I found that once I looked for it that way, so I could actually observe, then it became obvious.