Power Loss/surge @ 4500 RPM

[Australian ZX]

My fully imported 91 Twin turboed Zed (modified in Japan prior to importing) has suddenly developed a massive flatspot.

At full throttle the engine simply dies ( as if you've lifted ya foot) around the 4500rpm mark. If you lift ya foot back to around the 3/4 mark the engine gets stuck straight back into it and powers on......

On a couple of occasions there has been a quiet backfire during this 'flatspot', and yes whan I say quiet it can 'just' be heard !!

The engine is still reving out but only if you stay away from the full throttle position, this however has meant a slight loss in the acceleration/power the car had a few weeks back....... I've changed nothing, fiddled with nothing, in fact havent even had the bonnet open prior to the sudden apperance of this problem.

TPS and MAP/AFM have been swappedECU has been swappedAll connectors checked and cleanedWaste gates checked and cleanedExhaust checked for blockage

Any ideas ?

AU ZX