Could be a number of things. Most likely AFM or spark plugs.

If it's only happening on heavy boost I'd say the plugs.

I run a Unichip on my MY00 WRX, with a similar setup, but mine has the boost module, instead of using a bleed valve or anything. I had a similar problem to you last year, and thought it could have been a tuning issue, but turned out the plugs just needed changing.