Volvo: Turbo, b230ft, new spark

Question
I have a 88 740 Turbo Intercooler B230FT with 162000 Miles, when I'm driving the car will chok,shake and lossing power if I try to go faster. I noticed that whenever the turbo gauge get down close to the white area, I lose power and I have to release the gas and press it again.
I had checked :
Air Mass Meter,Ok
O2 sensor,Ok
cleaned throttle body
CBV,Ok
new spark plugs
New wires
Do you know what a turbo sounds like when it starts to die?
my guess is first TURBO or Fuel filetr.Any idaes?

Thanks
No black smoke

Answer
Masoud,
Test fuel pressure when this happens. Fuel filter could cause the lack of power if it's old replace it. Sometimes the seal in the turbo leaks oil into the exhaust, the bushings wear out and the impeller become loose and sort of rattles or scrapes,or locks up completely. Test the boost presuure to make sure that it is correct. (7psi max).
Check to make sure that all the intercooler hoses are tight and there are no holes that will cause pressure loose, that includes the intercooler also.