[jhv48]

If ever a car needed to be analysed on a chassis dyno, this is it.

After all you've done, it has to be either spark disintegrating or fuel leaning out once the secondaries open up. Your primary jets are fine as evidenced by the fact that it cruises just fine and your plugs are good. Problems only seem to crop up when you go to wot. I'm ruling out a broken valve spring for now.

Before you throw more money at it, try bigger secondary jets. Go up about four sizes and see if there's any improvement. One of those existing jets also could have an obstruction in it. Easy to pull the bowl and see.

It's going to be something simple, you just haven't found it yet.