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Power loss at>4800 rpm

My continued test acceleration runs in each gear have identified a commonality 
to my performance deficiency.  Full acceleration in 3rd, 4th and 5th gear 
suddenly stops when I reach about 4800 rpm.  The engine check light does not 
come on... though i may have heard some rattle = knock?.  
This affect occurs if accerating in one gear only or going up through the 
gears.  Its certainly disconcerting when passing and significantly limits 3rd 
gear's usefulness.  Ambient temperature 18-24 Deg C approximately 40% RH.

I have the Schrapknell Knobben (R) with maximum boost set to 16 psig.  The 
dash manifold pressure display reads a maximum of 1.3 bar from 14 psi up to 16 
psig.I also have Orins new stiffer wastage diaphragm (and Dunlop SP8000 for 
the 5th gear portion of the test).  I also have the TAP blue hose with turbo 
bypass (Bosch/ Audi part from S6).  Acceleration is strong - once the boost 
comes on until this point where the acceleration stops, though the boost 
pressure stays up (as monitored by test gauge in the glovebox(on hose line to 
wastegate and SK).  In the lower gears the revs will continue to rise though 
slowly but in 5th they hold steady.on a flat and only gradually increase 
downhill.  If I reduce the throttle to drop boost to approx 10 psig 
accleration resumes (though of course proportionately slower than before).

Before I increased boost I had the car tuned up new OXY sensor, NKG tripple 
copper plugs, new bosch wires, new distributor cap and rotor, new fuel filter, 
and I had my fuel injectors cleaned when out to install the new (rebuilt) 
cylinder head.

Orin suggested the computer may be selecting the lower octane ignition map due 
to knock detection.  If so why doesn't the engine check light come on and why 
does it just occur at high RPM?  The other possiblility is htat my engine is 
set up to run too lean?  This of course could lead to preignition as the 
engine charge temperature increased... but why at 4800 RPM.  The car has just 
been in the garage for 4 weeks so I don't really want to send it back in quite 
yet- enough comments from my wife about being stranded with me taking the 
Corrado for work.

My other ideas include: 1. Possibly a fuel starvation problem i.e more gas 
needed at higher RPM and higher boost?  If so how do I get more fuel in? can 
the fuel pressure regulator be adjusted or the injectors replaced?
2. Possibly pre-ignition due to temperature due to higher boost? Will I need 
cut down on the boost until I get a larger intercooler?
3. Some mysterious ECU function.
4. This is just the way it is unless I change over to EFI as per Graydon 

Any diagnostic assistance would be appreciated.

Thank you
Colin 86 5000 CD Quattro 308k km