Did you end up finding the problem?
Had something similar with my 95 Bspec Legacy, some days would go ok and others was way down on power. When playing up boost was very slow to build to max of 7 psi at redline even if wastegate hoses were off. Tried substituting everything poss could incl both turbos with no luck. Replaced engine after a "friend" ran a big end bearing and has been completely different car since. Far more power than had ever had in 5 yrs since I'd bought as a fresh import. Never found definite problem but ECV would be top suspect.
