Hi Mark

Your fault description suggests that the turbo is not spooling up & generating necessary boost at lower engine speeds. Perhaps there is a problem with the boost control system? Is it vacuum controlled or does it have an electronic actuator?

If its vac controlled, can you activate it with a Mitty Vac & see if low RPM boost is achieved?

You need to establish if the ECU is commanding an increased boost that is not being achieved, or otherwise.

You may have a subtle charge system leak, although unlikely seeing that the power was good for a moment.

I wonder if the new MAF is the correct one?