Why are the mufflers breaking?

I have a 1984 318i, and the muffler tore a hole right at the exhaust pipe. I ordered one online (a cheap one) and brought it to my mechanic. He said that my old one (which was oval shaped) was
something that had been thrown together, and that the new one, which was much heavier metal, was the correct one. He bought the hangers and put it on.
A few weeks later I have a hole in the new one, same place as the old. I can't understand why this would happen, especially since it seems like pretty rugged material, and it seems very straight forward to put this on.
Any ideas why this might have happened?
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Sounds as if something is out of alignment or the mountings are in the wrong place or the engine mounts are crap.
I would look at the engine and gearbox mountings first then go for the alignment and then the exhaust hangers.
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