My car holds boost at 12 PSI upto 4300 RPM, when I pass 4300 RPM, it suddenly drops with a jerk of the car to around 7 or 8, it then judders around up and down between 7 and 9 as if it's trying to pressurise but can't hold it's boost?
Any ideas why this could be?
It's got a blow off valve which was on it when I bought it, could this be a problem?