hey guys I have an ej20 turbo 99 gt forester, found this site to be so much help I thought I might post up a thank you, and what fixed the problem

for a few months now my foz has been intermittently surging on boost, massive flat spot from 0 to 8psi boost, run 15psi at 245rwhp normally. rough idle, on a wet road would throw up check engine light, other times run perfectly. no cell codes very odd i know
an ecu reset would always get it running perfect again for a day or two untill it rained again. would run perfectly on full power for months and then slowly deteriorate again untill a reset

found the ecu earth on top of the strut tower to be a little dirty, was an instant fix, after changing afm sensor and changing all the vacuum lines, engine runs a interceptor that can be easily removed and original harness plugged back in had the same fault

and Ive noticed this is a common fault through out the forum, clean the ecu earths