can u keep the car running without pressing the gas ?
if u can, let it run while u go around all the vacuum lines spraying them with carb cleaner. spray around all the joints, including the intercooler and tubing. when and if you find a leak, the rpms will raise and the idle of the car will kinda smooth out.
thats my easiest explanation, hope you understand it.
just the gauge off it happens all the time, your porobably still hitting 10psi of boost just the gauge is reading -2 at 0 so at 8psi its really 10