This is called boost creep, and occurs when the turbo is pushing so much air that the wastegate, even when fully open, cannot dump all of the intake pressure. This results in a continual increase in intake pressure, and is common with upgraded turbos, especially with upgradeddownpipes - the exhaust would rather flow through the turbo/exhaust than the more restrictive wastegate, which spins the turbo ever faster. Cars with this problem can develop mind-blowing (and engine-blowing) intake pressures in a hurry.
The general solution to this problem is to port the oxygen sensor housing, turbine housing and/or wastegate to allow them to dump more air. Otherwise minor malfunctions of the wastegate may also exhibit themselves as boost creep, such as poor travel on the wategate actuator arm. Some people use an external wastegate for better pressure control.
People interested in more theory behind this problem will enjoy Dennis Grant's Turbo Fundamentals Series.
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