When you first start, the ECU runs in open loop, ignoring the lambda sensor.
So, if its bogging it's putting in too much fuel
If it's hesitant then it's lean.
Too much fuel with a normal ECU is either wrong fuel pressure reg or rail plumbed has been plumbed in the wrong way.
The injectors have been changed and the wrong flow rate installed is a possibility too.
I'll bet whoever swapped the loom in, didn't include the wiring for the diagnostics plug or the EML either.
You might have a faulty TPS, but that wouldn't account for the sudden change at 4000rpm.
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