not boosting as much?

on my 1986 Dodge Shelby Charger...
Turbo 1
Stage 2 ECU(14psi)
Exhaust
Injectors
Fuel pump

So my boost guage is inaccurate(i know this for a fact, but it is consistent)
the guage was reading 20psi before... now under wide open throttle it hits about 15 and then drops to 10 and the engine noise changes...
i dont know how to describe it, it gets flatter sounding...
Other than that the car is running great.. builds boost quickly...
Is this maybe a logic module problem?
Or what should i look for?

[tryingbe]

The engine noise changed is probably because the waste gate is open. Sounds like you have boost spike.