Radium FPR violent shuddering in Return Line

Hey guys, I'm having an issue with my Radium Direct Mount v2 FPR I've installed on my Evo 8 / 4G63T.

The Radium FPR will cause violent vibration in the return line. Needle vibrating a lot on the gauge. Feed line seems fine.

https://youtube.com/playlist?list=PLH_BTPC6u4cs7ItGcD6m32lcLzT3CksWA

It's a Radium Direct Mount FPR onto a Radium Fuel Rail with small Radium Fuel Damper.

Walbro 460lph + 8AN Feed/6AN Return w/ 10micron Speedflow Filter. Have set base pressure to about 45-50psi (as per was set on the previous FPR).

Things I've tried:

- Hardwiring Pump On; issue still happens

- Replacing Pump; issue still happens

- Running low voltage; issue still happens to a lesser extent

- Hose out from FPR to bucket; issue still present (see YT playlist)

- Tomei FPR to bucket; issue doesn't happen (see YT playlist)

- Inspected internals, tried other supplied orifice; issue still present

- Adjusting FPR pressure/Running no Vac reference; issue still present

Radium have suggested I try a smaller orifice (not supplied) - which they have sent and I have not received yet.

I have also had it mounted on another Evo running a V1 FPR. Issue still happens on that vehicle. (Won't have the opportunity to try that V1)

Car idled a tiny bit more lumpy than usual when I decided to try turn the car on, but kind of seemed fine.

Ended up getting the FPR replaced (to rule out faulty FPR) and the issue is still present, however my car is very very lumpy on idle (no vac reference) and AFR Lamda is reading ~0.65. I'd like to believe that the fuel system is responsible for the car running lumpy, as it was running somewhat fine before and nothing had changed. (See attached video)

Will try run the car again tomorrow with the vac reference plugged in. Running out of ideas of what to check.

Car was running fine previously on stock hardlines/stock rail for context, before upgrade to AN Lines/Radium Rail/Radium FPR. Running an Autronic SM4 PnP ECU.

Any ideas on what to investigate to resolve this issue? TIA.

I was thinking a sticking regulator, but you've tried another one and problem still there.

That's a VERY rich idle lambda - even if you're running methanol which may in turn be causing swelling of seals that could cause problems? Having it around 1.0 is going to be better for the engine's ring' life, better for the oil, and should be better for the idle quality.

It may be some weird thing specific to your setup where the pump frequencies, rebound frequencies of the "rubber" tubing, lengths of runs, etc, have some sort of positive feedback, but as the first FPR also played up in another vehicle, I'd strip it down and check for wear patterns and any resistance to smooth part movement.