Oxygen sensor and high speed stumble

[The Thibodeaus]

I have an 90 Vette with a L-98 which above 4000 RPM loses performance and
it has been suggested it is starving for fuel.  The fuel filter has 500
miles on it.  I pumped some fuel out of the tank tonight after shaking the
car to check for any sediment and the fuel looks fine.  I am running an
adjustable fuel pressure regulator set at 48 psi.  I pulled the oxygen
sensor tonight and it had some black soot on it indicating a rich
condition.  Here is my question, by upping the fuel rail pressure to 48 psi
am I starving the engine at high RPM because the pump can't flow enough
fuel at that pressure?  Does this seem logical?  I increased the pressure
compensate for a ported plenum and also to overcome a pinging problem
(several prior posts made to this board and Vettenet on this nightmare) but
it hasn't worked.  I am working with the chip programmer to resolve some of
the pinging problems because I have found that they completely shut of the
EGR function.  So, should I lower my pressure?  Any suggested value?
Thanks for any suggestions.

Bob T