loss of power at certain speeds

All fixed. Turns out that the Dawes valve was faulty. They put a different one in and everything is back to normal now.

Would love to know how the dawes was faulty. There basically nothing in them to be faulty. A spring and a ball. And thats it.

I have had a faulty Dawes valve where the spring was not stiff enough and was compressing under normal boost pressures.

Thats not a faulty valve. Its just some engines require a stiffer spring to work the same as another of the same engine. That issue had been resolved now with a stiffer spring in all valves.