X2 on the boost dump valve on the intake. It blows when boost goes much over 10psi, and shuts again when well below 10psi. It did this on mine when it had been laid up for a few months, the wastegate on the turbo had siezed so it was on full boost all the time. Car would go like a scalded weasel then "poosh!" flat as a fart for a couple of seconds, then off it would go again. About a three or four second cycle. Fortunately it freed up after a couple of miles. Check the turbo isn't overboosting too.