I would point to one of two possible things....

1) boost limiter. My last S1 was like that, floor it nad it was hesitant and sluggish, lift off a bit and it flew. Problem solved but finding out someone had wound the actuator out way to many times so wound it back in

2) ignition problem. If not a boost issue, only other ting I can think off is maybe a plug(s) not sparking properly around that rpm range. Forthe sake of £15 might as well change the plugs....