Hey Bruegger,
I was just wondering...Oh, welcome to the group, btw
...But yes, I was wondering if you have problems with your spine. I also have Eustachian tube dysfunction, TMJ as well as MG. I have some problems with my spine (I just started a thread about it). I noticed that if I turn my neck in certain positions, sometimes my jaw will improve a bit (the stiffness), and fluid will actually leak out of my ears! I also found some info. stating that having reverse kyphosis can reduce breathing capacity by up to 30% because it puts pressure on the glossopharyngeal area, so it's all upper respiratory. Not sure if this is where you feel the breathing weakness, but it is for me.
A year or so ago, my dentist made a mouth guard for me, and he said that this can help in correcting spinal issues and therefore increase breathing capacity and improve sleep. You might want to check it out. I haven't used mine in a while, but I plan on starting again.
I hope that you get things figured out soon!
Nicky