Stock Turbo, 142000 miles.
There's your problem. By the way you are describing it, it sounds like excessive shaft bearing play on that sucker. You can check it by removing the air box/MAF/accordian hose that goes onto the compressor housing (inlet) of the turbo and with your index finger and thumb try wobbling the compressor wheel shaft. The spec is something like .13 mm so any play at all is not good.
Only other way you won't hold boost is a vacuum leak. Try checking the vacuum hose that runs to the wastegate actuator.
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