So this is basically cruising. Yes?
The motor is at 3100, car going 75 mph in 4th.
So a test to determine if it is rpm related is to take it 70 or 75 mph in 3rd. That you could still safely do on a public road.

Testing fuel pressure while driving takes a little bit of effort to arrange a visible gage safely. Lots of option but take your time on doing that right. Cost of an error is too high.