What is your base fuel pressure set to on the
fpr?
You said in your first post you were targeting 350kpa fuel pressure under load. That sounds too low.
Standard Subaru base fuel pressure is 43psi (630kpa). You would want to be close to this as a minimum starting point from a safety/tuning point of view and then tune/adjust as required.
As GtLeggy has said above the
fpr should keep fuel pressure steady.