Iggy beat me to it - weak mixture.
Does your 2.5 run on carbs, Lucas Pi or modern EFI witchcraft?
Pi tends to run lean at the top of rev range, so should be SRT a bit rich at idle etc.
The details are in the Lucas Pi manual, but you might do best to send the Metering Unit to one of the specialists to be recalibrated.
Prestige and the other one have calibration rigs, just tell them about your engine and how you will use it and it'll come back perfect!
If carbs, set them a bit richer, if EFI ditto, at the high revs where you're going weak.