Agreed, most likely the fuel filter / fuel supply. Does the idle hunt a little when you come off of the freeway or is it a strong steady idle? Fuel starvation can be from the in-tank filter, the small strainer-filter, or the main filter on the engine.

The wastegate on your car is direct (pressure) actuated, not a very common failure.

Boost enrichment described above will significantly reduce your power to that of a non-turbo. The line from your intake plenum runs across to the bottom fitting of a "switchover valve" near the windshield washer bottle, then another line runs from the middle fitting to the injection pump's ALDA. Failures in this are common, usually the switchover valve, the hose, the restrictor/orifice, or the fitting on the intake become plugged with soot. This can usually be blasted out with WD-40 or another mild cleaner/solvent. The switchover valve is normally open and you should be able to blow through it. Also test to be sure that the ALDA will hold pressure (~15psi/1bar), if it leaks it might not allow boost enrichment. I have not seen this reduce power enough that your car will not shift automatically however, it should still reach 5250rpm no-load and well beyond the shift point under light load. Not reaching this RPM indicates more likely fuel starvation, the engine grows quiet and has no smoke, will not run high enough RPM.

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