hi, i have an audi 80 tdi which had the same problem under load at higher speed, this reported a code of 00575 back on diagnostics, which ended up being the sensor on the intake to the turbo, this is the one with a plug on the underside with a red pipe that goes under to the turbo and there are two other small diameter pipes on it as well. genuine part was 36quid plus the tonic.

hope this advice is some help to you.
mark