iv'e noticed this a couple of times .the causes where _
1. loose intercooler hoses or a vaccum hose come a drift
2 . boost controllers - if you use a turbosmart gated or dual stage or any similar pressure relief sysytems the wastegate can be locked open by boost .ebc of any kind wont cause this
3. change your plugs use NGK bkr6es .if you use a fuel cut defender make sure the boost cut isnt set any higher than about
18 psi .otherwise you can effect the map signal too much .
My wifes Cressida behaved similar to this from the same thing .boost cut was set to 20 psi and the car ran too rich .and was VERY laggy (after a boost run)