'85 12A Exhaust Issues
 

My car keeps burning out the collector gasket at the end of the exhaust manifold every couple of months. I'm thinking it's some kind of mixture issue, though I haven't gone through the effort to go chase it, so I'm not certain. I don't have any performance issues other than that, so I'd guess that the mixture wouldn't be far off if I'm right. As far as I know, the car has stock ports, and I know for sure it has a Nikki with the rat's nest intact, with stock ignition except for a couple of MSD coils. I run it on 87 octane without premix with Royal Purple 5W-30 in the sump.

If it keeps burning that same gasket, I'd bet the mating surfaces are not true making for a spot that has little tension against the gasket. Either one of the mating surfaces is warped, or the pipe/manifold aren't lining up quite right.
If the gasket is always burning out in the same basic spot, start looking in that area for warped surface(s) or misaligned pipe.

Misaligned pipe would probably be it. My little rubber exhaust hangers are starting to wear out, so the cat/midpipe section is too low.

Are you using quality gaskets? If so Roger's advice is probably a good place to look.

Autozone gaskets, your mileage may vary. Thicker than the normal piston motor collector gasket with what looks like a metal o-ring around the inside of the exhaust hole.

I wouldnt trust anything but OEM Mazda gaskets on the exhaust.

just jerk it off and place a header and straight weld it to a pre silencer and a good muff
take the cat to a scrap metal dealer and get some of your investment back