I've heard of this happening a lot with the EJ25 and although a reflash or stand-alone is the best way to go, there is a poor man's fix...allegedly. Bore out your wastegate passage. I don't know anyone who's tried it, but jscspeed.com says it works:
"04-06 STi OWNERS READ: On the 04-06 STi there are boost spike or creep issues in all cases with the use of an aftermarket downpipe or turboback that uses a bellmouth or external dump tube design. This is because the 2.5L engine pushes so much air. Even with the internal wastegate fully open not enough air can exit to stop spinning the turbo, the result is boost creep or boost spike. You can fix the problem by porting the stock turbo's wastegate passage (any exhaust shop should be able to do this or it can be done at home) or you can get an engine management system to fix the issue, like a UTEC or AccessPORT."
Hope this helps.