Earlier in my S1 efi I came to a dual carridge road and as i went to join I sped up in third and the car hesitated jolting I went to fourth and as it boosted did the same thing and in fifth I could only get to 45mph until it evened out then was ok without boost.

Came of the dual carridge way and tried to speed up on a single road to see what happened, did it in third but after 2 miles it was ok on boost again.

It does this in 5th gear if the cars at 70mph and I try to boost it

What's the problem?

 

Sounds like an air leak to me, had a similar thing an it was the bottom hose on the intercooler