I was having very similar issues, turned out to be the seal around the EWG, my boost ring thing was loose. Feel the turbo for play, check intercooler pipes and joiners, check core for leaks aswell. Could whip off the decat and look for fucked oil seal. You can buy EWG blanks on ebay which you attach to the mani instead of a EWG, it will rule out a faulty EWG as it bypasses it, but you will technically be running unlimited boost whilst testing.