Low Power at High RPM

[runningonmt]

I have a '93 964 C2 with stock engine, ECU (no performance chip) and ignition.

It runs well under partial throttle, but when at wide open throttle, it hesitates at ~4800rpm, then continues at a much lower power level than I'm used to. This started happening on the last day of a HPDE at VIR. At several marks on the back stretch, I was 5-7 mph slower than the previous day. At home, I've been testing on freeway on-ramps and the issue occurs in both second and third gears. I've tried a number of things: drained the fuel and re-filled with fresh Shell gas, replaced the fuel filter, swapped out the DME/ECU to my spare, checked the secondary distributor belt (its there) and checked the function of the resonance flapper valve (rotates on startup).

What steps would you recommend to trouble shoot this problem? Any specific areas that you would suspect?

How do you determine that the phasing of the two distributor rotors is correct? If the fuel pressure regulator is the problem, can it be replace without removing the intake manifold?

Thanks,
Rick G
Richmond, VA