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Use your O2 sensors to monitor fuel ratio during the flat spot.
Have you inspected the DFI plug?Update:
Curiosity got the better of me,
I replaced the DFI this afternoon with a new OEM one [Delphi model], that I had on the shelf as a spare for a rainy day,
And waited till the engine was cold from this morning,
didn't take long though, and it felt like 8deg most of the day.
And you guess it,
The Damn Flat Spot is still there when cold under acceleration.
Oh well, one would say, back to the drawing board.
Have you inspected the DFI plug?
Have you inspected any part of the DFI wiring loom?