The problem is almost always the USE of a thick 'doughnut' gasket rather than a std 3-hole sheet gasket. Due to the design of the Pantera exhaust system hanger support plus the wt of the assembly, there is considerable force directed back against the header flange. This force along with heat & road vibration eventually destroys the crushable doughnuts. But due to its thinness, a sheet gasket allows the force to go thru it without collapsing. Of course, sheet gaskets won't compensate for much mis-alignment nor grossly non-flat surfaces as well as a doughnut, so you takes your choices...My best guess.
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