Lean mixture at high revs.

[FatBoy]

Hi,
I have just had my Cobra on a rolling road in order to address some minor carb related running issues. The engine was built by one of the respected FE builders and has been installed without any subsequent modifications.
Although there is nothing major wrong with the engine, it has never run as well as I feel it should do.
The maximim power at the wheels was 348bhp, but the curve started to get very lumpy above 5,000rpm and the air/fuel mixture became lean. The engine was quoted as having significantly more by the builder and the spec would suggest it should have according to Desktop Dyno.
I have the Holley Red Top fuel pump as fitted by Kirkham, running in series with a Carter mechanical pump as fitted by the engine builder.
The lean mixture at higher revs would suggest that not enough fuel is getting to the engine when the demand is high and while I was out in the car yasterday, I noticed that under hard acceleration, the fuel pressure would drop significantly, which seems to confirm it.
Many Kirkham customers run much bigger and hotter engines than mine, so I'm assuming there isn't a fundamental issue with the capacity of the Holley pump and the fuel pressure under normal running conditions stays at around 7psi.
Can anyone shed any light for me?
I'm sure additional information will be required in order to help diagnose teh problem, so please ask.
Thanks,
Paul