If the true problem is overpressure (vice a bad sensor), perhaps it is related a faulty pressure relief valve, allowing overboost?
Since the car's performance is unrestrained, it suggests you are getting (at least) full expected pressure in the intake.

If it were me, I'd be rigging in a pressure gage on the boost side to verify actual manifold pressure.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles