A bad header gasket will have a minimal effect on performance, at first, but will soon lead to a burned exhaust valve in the cylinder where the gasket is blown out.
The only reasons a header gasket will blow out are loose bolts or an uneven surface. If the bolts were always tight, then either your head or the header is warped slightly. Odds are it's the header.
Are you positive it's the header, and not some other part of your system, like the plugs at the back of the heads?
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