Had the car at another tuning place today. They found the fault thats causing the flat spot. Its the accelerator pedal module.
Its loosing voltage from 5 volts down to 2 volts during the acceleration between 3500rpm and 4300rpm.
Pic of pedal module on Realoem
Finally the flat spot has been resolved, after months of being at a few places to find the fault.
Thanks for your help guys, for what you suggested I should try