I have an Audi which does the same thing. It is normal to spike to 20psi then fall back down 14-16psi and higher rpm. I was told is does it because the car is leaning out and cant maintain full boost. It is normal to me and i leave it chipped also do u have other mods. B/C with the audi i have all the mods and front mount intercooler for it and it runs pretty damn good. ONe more thing APR has a stage 1+ which includes bigger injectors which gives the 1.8t better power in higher rpm's