I have LEMS and mine does the same thing Weak voice after a few sentences.
Also i get a weakness in my chest makes it hard to breathe also if i sit leaning forward the same symptoms equates to weak diaphragm muscle due to the LEMS.

my other weakness is all over.
I refuse cellcept or imuran im not risking 3 to 4 types of cancer when we are trying to be cautious and checked for cancer every 6 months with LEMS..
I finally found relief for a few hours at a time with the
3,4 Dap and the mestinon
neither ALONE did anything for me at all

Now i take 20mg 3,4 DAP with 60mg mestinon every 4 hours and i am almost able to get up off a chair 60% of the time without using my hands.

I am also able to breathe easier while the stuff is working
yes it does wear off and in the mornings i am so weak i am almost unable to even lift a coffee cup.. but such is life with LEMS..
Stay positive and eat right that's about all you can do,,, and never give up.