I had a completely stock WRX that some cock had put an extra olive/restrictor pill into the line coming off the turbo, unlikely but worth a look, could also have the STI olive in the line which is slightly smaller and will give higher boost, seems like a good idea at the time but can cause overboost/fuel cut.

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Beaten up old 98 Gen 2 Liberty, 13.0, stock V3 STI running gear.
07 Transit motor home
69 VF & 70 VG Valiants needing full restoration
Yamaha Zooma 50cc rocket hopefully going with the Transit.