audi a3 2005 limp mode issue?

[X57E]

i have a 2005 audi a3 2.0TDI, usually it drives great and gains speed quickly when i floor it but if im on a long drive and i come off the motorway and hit a red light, when i go back onto the motorway and put my foot down it struggles to get past 40, taking my foot off the gas and then slowly accelerating i can get up to 60 but verry slowly, the rpms seem to stay very low. turning the car off, waiting a couple minuets and then going again will fix the issue temporarily untill i go fast again and slow down then go fast again, but sometimes it does not go back into what i believe is "limp mode" at all, its very random sometimes, i assumed i needed my oil change as it was overdue from the last owner, i changed the oil filter and oil and it was fine for a week but it started again today, it ran perfectly yesterday on the motorway for 2 hours, any advice would be greatly appreciated, the car has no warning lights at all when this happens.

[Steve Q]

I'd recommend a diagnostic check to see if any fault codes flag up. 

Limp mode would come with an engine management light. 

It could be an issue with the turbo