I too had muffler issues until discovered the cure here on a forum. Here is the solution that works for me.

I do not use a gasket. I clean up both mating surfaces well and then clean them a final time with acetone. I then apply a very thin layer of Permatex Ultra Copper Sealer. I also put a dab on the threads of the bolts that attach the muffler (acts like a thread locker). Gently assemble the muffler to the engine and finger tighten the bolts evenly. I then torque it down tightly and let it sit for a day.

For me there is no more loose muffler and no leaks.