Has the turbo intake pipe been changed or is it the standard jobby as the originals get soft and collapse.
Also you could try running the car direct and bypass the Amal valve to see if that's faulty but be very careful when doing this.
Failing that check all boost pipes.
Could also be a faulty dump valve if it's the standard one, you could blank it off to make sure it's not that.
I had the same issues your deceiving and mine turned out to be the vacuum hose to the dump valve was kinked and the boost pressure at that rpm was opening the dump valve early.