Losing power

[Adrian Cox]

Hi all , I'm after some trouble shooting ideas . I have a 57 plate Audi a3 sports back 2l TDI auto , I lose about 50% of power when engine is warm and usually when it's traveling half way up a hill. I have taken it to my usual garage who could not find anything wrong with turbo , so we changed fuel filter  this has made no difference , any ideas please .

[Trevor]

Does the engine management lamp come on at any point?