I'm going crazy here. I have a standard setup, had my head rebuilt and the engine sounds silent like it did over 15 years ago

When I floor it, my boost gauge shows the turbo wanting to boost to 1.1 and sometimes 1.5 bar and obviously cuts out. I expect it to go 0.9 then to 0.8 as it always has for over 18 years. Is this the behaviour of a faulty Amal valve? What's an alternative to an Amal valve, struggling to find a new one. Boost control kit?

I also might have a secondary issue of slight hesitation after 4k rpm, not sure if they're related but need to address boost issue I think first.
George