Ask your tuner to tune it in open loop boost control and see what it does.
I've had similar with a car, but it was due to a lack of boost pressure sensor.
You need to drill out the small return hole in the return venturi that pops into the bottom of the mra to prevent excess fuel pressure.
Any of the half decent pre assembled 460lt/hr mra's should have the bigger hole already.
Spotted @Boganspeedon the way back from tech at the plex.
Was there with a couple of the usual suspects drinking coffee and looking at cars.
The Army Tank got to hang out with its great grandad and its annoying American cousin.
Give It a decent whack on the face of the plug first.
It's also good practice when trying to remove steel parts that are threaded into ally to tighten them slightly first then back them off.