Code 559 ISX15 2350

I'm getting an intermittent 559 even at bobtail when I'm running it hard.
Fault Code: 559
PID: P157
SPN: 157
FMI: 18
LAMP: Yellow
SRT:

Injector Metering Rail 1 Pressure - Data Valid but Below Normal Operational
Range - Moderately Severe Level. The ECM has detected that fuel pressure is
lower than commanded pressure.

It'll run good for a few miles with boost hitting 33psi...then it'll flash a check engine for a few seconds and go away. As soon as it does that, I'll lose torque and my boost will only get to about 22psi.
When we check it in Insite...this code is present but INACTIVE. But the de-rate doesn't clear when the code goes inactive?

We've changed filters, tested injectors, changed relief valve and fuel actuator. Also, swapped fuel pressure sensor.
Had a decent amount of bigger air bubbles (under throttle) when checking with clear hose so we bypassed the F/W separator and bubbles stopped.
Took it for a another drive and same thing.
Engine was rebuilt about a month ago and high pressure pump head was rebuilt at the same time.
We've checked all we can think of so we're going to pull the pump head and take a look at that again.
Any other ideas that I should check?

Edit: Also, does anyone know what the maximum commanded pressure should normally be at full throttle/full boost?

 

They did leak down and some other test (can't remember).
They're new injectors that were put in at rebuild but one did fail so we replaced it and then it tested good in shop.
But on the test drive we still got the quick 559 code and de-rate.

 

We did rebuild the pump with new plungers, rollers etc.
But we're gonna pull the top again and make sure everything is good. Maybe something is sticking in there.

 

I'm not sure...I wasn't there for the testing. We're going to test those plungers tomorrow and if they're good...I'll probably end up taking to Kenworth this week to do more accurate testing
It would be there already but weather has been nasty here for a week or so and the dealership is 150 miles away.

 

Took pump head off and had a broken washer. Ordered the parts and hopefully have it back together tomorrow morning....and working