I have an MY99 Turbo which has developed this problem.

Basically, the car pulls well to the redline through 1st, 2nd and 3rd but sometimes in 4th and always in 5th, when it reaches around 5000-5500 revs, the car nosedives and looses all power for a couple of tenths of a second, then the power is restored???

I've checked all the hoses for leaks, changed the fuel filter, filled it to the brim with Tesco99 but the problem is still there.
Plugs were changed 6mnths ago.
I'm thinking of changing the plug leads next.

The car has an STi8 intercooler therefore I had to cut down the 3 bolt points where the distributor sits and now it rocks slightly even though the bolts are tight, also the check engine light came on the other day and the faults which showed up were the Maf and O2 sensor.
Could my problem be due to any of these faulty sensors?
I reset the ECU tonight but the problem still persists.

All suggestions welcome.