Audi A4 1.9 TDI losing power on motorway

[Walter Sobchak]

I've recently got an old Audi A4 which had been sitting for a while, it seems to be fine around town and pulls ok but on the motorway as soon as you get to around 3000rpm in 5th it feels like something cuts in and it hits a brick wall in terms of power.
After coming off the motorway it and driving around town again it feels like it's in limp mode and not performing properly, if I turn the ignition off and then on it feels ok again.
Does anyone have any idea what it could be?.