Problem after turbo replacement.

[Little Old Me]

We have recently replaced the turbo on our Sharan (2002 115 BHP TDi) and have a small problem. Every now and then when we change up (usually 3rd to 4th) the car hesitates before picking up again, it is only for a split second and twice it has dropped the turbo and logged an error code of turbo underboost.

Any ideas guys/gals.........