Hi folks

Well I got myself back into a subaru today and I have issues already!
I got the car checked over by a well known Scooby mechanic in Ireland.

I got a 99 B4 twin turbo legacy, The guy I bought it off has just rebuit the engine, I was on the way home, was driving easy off boost for ages and decided to plant the foot in 5th for a min! The car got to about 3700rpm and then wallop, like i hit a brick wall. Backed off immediately, slowed down and tried again in 3rd, wallop again same revs.
I slowly brought up the revs and it didnt happen at all at half throttle.

I dont have a boost gauge, but the mechanic reckons over the phone its a "plumbing issue" something to do with either mis fitted boost solenoid pipes or dodgy plumbing??

Anyone shed any opinions on this?
where do i start?