The top and bottom of the Gold Bars have Z-Fighting while the vanilla Iron Bars don't.

Quark Version: 3.2-356

It looks like the reason this doesn't occur with Iron Bars is the top and bottom is slightly inset from the edge of the bars, while the Gold Bars are only inset on the top, and not the bottom as shown below.

Iron Top:

Gold Top:

Gold Bottom:

Strange, since we DO offset it...