This is kind of a tough one because it could be a few different things or a combination of things. I know sometimes it is frustrating trying to figure out the issues with senior's. I am sorry you guys are having a hard time.
Anytime a dog has a "falling out spell" it is because of decrease of oxygen and blood to the brain. It can be many things that cause this and some symptoms can come and go.
In saying they did blood work and determining a thyroid problem I am guessing they did a T-3/T-4 test on her. It is true that a thyroid problem can cause all of the symptoms you have described. It affects the over all body and can also cause sezuires. Now not all sezuires are the ones that people think of the jerking around, she could be having mild ones. Thyroid problems will affect many parts of the body.
With Thyroid some of the most common things are:
Becoming lethargic, weight changes generally gaining weight, skin issues dry skin flaking scales and dark skin... allergies and skin infections, hair loss, lack of luster to the fur, color changes to the coat, lack of tolerance for the cold, behavioral changes, including fear or aggression, slow heart beat and decreased metabolic rate, seizures, stroke, neurological diseases such as partial paralysis or signs of senility.
In addition to this they can also present some of these signs with the start of kidney issues and/or kidney failure. In the blood work do you know if they did a BUN test? This level would indicate a problem with the liver/kidney.
It can also be heart related, the heart not working and pumping as it should. This is normally also seen with open mouth breathing and the chest showing hard breathing, like huff and puff to get in air. Panting when they have not been heated or just laying around, always seeming hot but not fond of getting cold.
All of these things are age related issues. The heart, liver/kidney and Thyroid are things that need to be caught early and treated. But keep in mind that it also takes about 60 to 65 days for the thyroid levels to level off with the introduction of thyroxine. I generally have thyroid retested at 30 days and 60 days to make sure the med's are working right and the levels are adjusting. I dont know the size of your dog but maybe she got the football shaped purple pill??
So, if you have another event then I would suggest further research into blood work on testing her kidneys and liver and then from there an ultrasound of her heart and lungs.