Try the gasket, make sure all the bolts and threads in the engine are clean of oil and contamination. When installing use a good quality loctite and let it dry.

What happens the gaskets expand and contract, the free play lets the bolts loosen up. Also remember that you have two different metals that expand and contract at different rates. This also allows bolts to loosen.

keep an eye on th the bolts and make sure they are tight. don't just set them and forget them.

Why you have a leak at the engine could be of several issues. The primary would be that the mating surfaces are not flat. If you overtightened the muffler it is possible that it is bent at the mating surface which would cause the leak. The muffler only need to be tightened until its snug, not to the point that the bolts won't go in any more.

Take a square before you mount the muffler and check the mating surface to see if its bent. There should be no visable gap.

Let me know.

Al