Hi I have a problem where my car seems to be holding back some days in all gears and others just 3rd and 4th. Nearly always 3rd and 4th gear. Seems to have plenty of power in 1st and 2nd gear to pull away but then meh. Also not sure if it's normal but anything below 3k is useless for accelerating.

Example

Pull away smoothly in 1st and get up to 3k then bam lots of power

2nd all power 3k+

3rd flat spot st 2.5k and max power from 3k+ but seems holding back

4th flat spot 2.5k and max power 3k+ but seems holding back

In the 2.5k flat spot the exhaust makes very deep drone noise

I can cruise no problem below the 3k power but it struggles to get over the 2.5k hump I'm sure this car has more to give

What engine is it.

it's had a replacement engine from an abarth 595, with vl36. Effectively same setup as 1.4 tjet just potentially more power but running on original wiring, sensors, ecu etc. Had same problem before engine was changed. Engine was changed due to bottom end knocking and suspect head gasket.

Have you got any fault codes.

Only brake light switch, seemed to work ok but replaced anyway. Will re check on the snanner tomorrow but made no difference. I can post a few screen shots from live data if that helps, pressure readings, actuators etc?