With no diagnostic codes, going to take some tests & experimenting. A fuel pressure test configured so the gauge can be observed while driving might provide a clue. Likewise monitoring the battery voltage while driving. And the intake manifold vacuum. Testing the calibration of the throttle position sensor makes sense. EGR problems could cause this. See if it makes any difference if you push the throttle all the way to the floor vs not quite all the way. The EGR is supposed to automatically turn off when it is all the way to the floor. A faulty cat is another possibility. A shop with the proper scan tool could take a look at the pre- and post- o2 sensor signals, might indicate the cat was on the way out. Bypassing the cat (as a test only) is another idea. A fuel trim measurement might be useful too.
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