Early boost cut off

[boostedlegacy87]

I have a 91 Legacy Turbo. I was driving this road the other night and I was pushing pretty hard up this hill and my turbo was at full boost, and then randomly (without me changing gears or letting off the throttle) it cut off boost and dropped from 5 BAR to 3 BAR. What could be causing this? Is this strictly just that wastegate or is it in the vacuum system?

 

I'm looking at doing a WRX top mount which would mean I would have a different BOV setup correct? hopefully that would solve my cut-off issue...