When you hear about a "flat" cam, we are talking about a cam lobe that isn't as high as it should be and doesn't open the valve as much as it should.

No, you cannot swap flat tappet lifters around from cam to cam. Once the lifter and cam are broken in, they are matched by lobe and lifter location.

Like everything else, there are exceptions for mega-expensive race cams and roller cams but that's not what we're talking about here.

Like the man said, start pulling plug wires and see if any of them cause the engine to run worse, or more importantly, no different. The ones with no change are having a problem.