Open loop at 70% or greater throttle and mid-high loads there are no triggers specific to boost but more calculated load and/or VE. Part throttle boosting is still closed loop (such as being lightly on the throttle going up a hill but the turbo is chugging away). Disconnect the O2 sensor and restart the car to force open loop operation.
Your problem sounds more like a big boost leak or a bad AFM. Check for diagnostic codes stored in the ECU, see the MKIII FAQ section for details.
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