It really is quite simple, even for a mechanical numpty like me.

I also had a problem with my coop not liking PRV's and overboosting to 1.7 bar. When H2ypr fitted my Greddy (thanks Ross ) he also changed my boost hoses and boost has been rock steady since. We were both convinced that one of the hoses was split somewhere.

It almost certainly will be something simple and inexpensive (for a change ), it's just a process of elimination.