Octavia Vrs boost problems

[Stretch20v]

Hi all, been trying to fettle my beloved 02 vrs for a year now and wondered if anyone can help. On starting the car it boosts fine for a while with plenty power and you can hear the turbo spinning up no problem but it will with no apparent reason stop giving so much boost and if I switch the key on and off it returns to full right foot happiness! The n249 n75 and dump valve has been changed twice now and one other valve I can't recall the name of. Two garages have tried to fettle it now. I just need someone to say "hi mate I know exactly what I'm doing with these engines" or recommend a garage near me. Other than giving it to a Skoda garage and facing a huge bill I don't know what else I can do. Car has a full service history and has been cherished all it's life so this is such a shame.