Lost boost
 

So a friend of mine bought a imported 89 Fairlady Z32
Has upgraded turbos, an AEM infinity tuner, and AEM bost controller.

Was developing about 20lbs of boost max until recently when it cur down to just 9lbs and won't go any higher.

he runs E91 ethanol free gas
uses 5w-30 oil
Air system was vacuum checked and came back with no leaks
No coolant leaks at all.

only really weird thing is that when the engine is cold boost seems to be fine but once warm it stops and goes down to 9lbs again and won't go any higher.

any ideas of what could be causing this? Bad sensor somewhere?