I replace my exhaust manifold gaskets about every five years. What could be causing this and what do most people consider the best gasket to use. This is a 1976 F-250 4 x 4 with a 360 engine.
The next time that you pull the manifold, I would recommend checking for cracks and signs of warping. Perhaps the hardware is old? Something is allowing your gasket to become overly heat-soaked. You would probably be able to resolve your situation by purchasing a new manifold, or header, and having it ceramic-coated. Assuming that your manifold is in good condition, before you purchase anything, see if you are able to find a dead soft aluminum gasket. This would be the lowest cost resolution to your issue. Good luck!