Tim,
Thanks for the post.
The syndrome you describe could be coming from many things, but it needs evaluation. Many potential conditions can cause episodic exhaustion -- slow or fast heart rates, hypothyroidism,
dehydration, low blood sugar, anemia, heart failure, etc. Sometimes it helps to really be a strong advocate for yourself, which at times means pushing your doctor to investigate further. Uncommonly, it means that you have to seek second opinions.
Best of luck.
In searching for the root cause, if you haven't had and FSB reading, you might do that right away.
Regards