Is the turbo still std internal wastegate? If you have improved both intake and exhaust flow, are you sure it isn't spiking because the wastegate just can't pass the increased exhaust flow?

After full exhaust and intake mods mine would start to spike from 18psi til it hit the overboost cut-out, it just couldn't pass the excess exhaust gas through the wastegate and so it went past the turbine and created more boost, therefore more exhaust gas, vicious circle!
Ended up pulling the turbo off and porting the wastegate for high flow a couple of mm and all sorted! Would hit the set boost level and stay there solid as, no more spiking..