Turbo (pre) detonation

[Ade]

Hi, I hope someone out there can help me. I have recently brought a spider turbo and I have a problem with (pre) detonation when applying a small amount of throttle. I have attached a pressure gauge to the system and it appears that I get the detonation when the inlet is around atmospheric pressure (no boost). The detonation dissapears at mid to full throttle.
As there are so few of these turbo cars in Europe I cannot find anybody that has had a similar problem.
Could these be due to a faulty Lambda sensor or Enrichment switch?
Thanxs
Ade