Boost cut?

Hope somebody can help... My car should be able to handle about 24 psi, but when I get to around 18-20 I get a drop right back down to about 10 psi.

I was told by the previous owner that I've got a solenoid box from a lower spec legacy (GT, not GTB). Is anyone able to shed any light on whether this is the cause? Or is it something more sinister?

Thanks in advance