On a SS freewheel bike, what would cause such a difference betweeen the tightest part of the chain and the loosest part of the chain? I've tried different chainrings and freewheels so I'm guessing it's either the loose ball bearings, or the cones, or the hub itself?

When the wheel is coasting there's also a bit of "pulsing" in the chain -- going loose, going tight.

Thanks!