A3 2.0t fsi 4000rpm issue

[Slewz]

Hi looking for any advice on my car it’s a 2006 56 a3 sportback 2.0t fsi and every time it hits 4000rpm the Rev counter jumps up and down the epc and engine management light flashes the car feels like it hits a limiter I’ve removed a forge diverter spacer and swapped out the diverter valve to a revision D model, can anyone shed some light?

thanks in advance