Mini Cooper jerks when pulling away

Jerking when accelerating

Hi Guys,

my Mini is causing minor problems and my Mech and I are stuck. Maybe you can help:

Cooper S 170 S (series) + JCW mapping, 380 nozzles, camshaft, pulley, sports exhaust from Kat, no more pre-cat, JCW spark plugs. You can only drive with Super Plus or 100.

Consumption around 11 liters.

For a good two months, the good one has been jerking when accelerating hard under full and sometimes also under partial load.

If I accelerate at around 2000 revs in the same gear, it jerks once at just over 3000 revs and then again 2-3 times at up to 6000 revs. It feels like brief misfires, lasts a maximum of half a second, then it pulls well again. Performance should be there, feels pretty tight.

Ignition coil and plug were only replaced, candles are approx. 5000km old.

It is not always, but almost always, when you drive the Mini really hard. If you brake hard and downshift in heater mode and then go hard on the gas again, then it jerks briefly, looks as if it were only walking on three pots for a second, then also has such vibrations, but then comes back right away and pulls through cleanly until the next jerk.

Error messages are only stored by the lambda probe, but may be due to the missing pre-cat.

Do you have any ideas how I can proceed to find the error?