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I can't add much to what's been suggested, but there are three, and only three, possibilities.
1) There is something wrong with the "pump" -- if the pistons go up and down, the valves open and close the right amount at the right time, it will produce steady vacuum. Apparently we can rule that out at this point. Was a leakdown test done?
2) There is a leak on the intake side of the "pump" -- air is getting in someplace that's not through the throttles. What sort of crankcase ventilation system is in use, and how is it hooked up? I'm sure the integrity of all hoses and gaskets has been checked, but what if they're not going to the right places?
3) There is blockage on the output of the "pump" -- the exhaust. This would result in low vacuum, but steady... unless the blockage was in one or more individual runners of the manifold.
That's all it *can* be. Ignition and/or fuel problems cannot cause the severe fluctuations you describe, although they could contribute to milder symptoms. Something quite basic is being overlooked. I wish I could divine what exactly, but it's in one of the three areas listed above.
And no, I don't think a carb change would do a thing for it.
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