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Try running the engine without the FPR at all. To achieve an accurate pressure regulation at such low pressures requires a balanced diaphragm regulator, not unlike that used for acetylene regulators. One that costs $25.00 retail can't be well made, and I have never seen a good one. The HIF carbs are capable of handling more fuel pressure than the HS, so it just might work without it.
As to the power loss, sometimes bits of gunk end up in the float chamber and will intermittently plug the pick-up tube for the main jet. That shuts off or reduces fuel flow in one carb, giving the symptoms you describe.
If I had a nickel for every time I cleaned an HIF carb that someone sealed the chamber bottom with silicone, well, I'd retire....that's the usual source of gunk in my experience.