Has your vet ever done a complete medical work-up with blood tests and such?
Honestly - my very first thought is cardiac but I am not a vet and could very well be wrong. When she has these weakness "spells" or when she is doing the whole staring at the floor thing - does she pant?
I think I would start with a complete medical work-up. You might want to do a little research online and see what genetic issues are common in the breed and see if any of them sound like what she has so that you have some specific questions to ask your vet.