Does anyone know what could possibly be wrong with my dog?

I have a 5 lb, 12 year old chihuahua. Recently she has been very sick, with diarrhea and vomiting. After many many ER, hospital and vet visits she seems to be ok, I am just thankful that she is still alive. However, it seems that now she is having a serious breathing problem. She will do this thing when she is... show more I have a 5 lb, 12 year old chihuahua. Recently she has been very sick, with diarrhea and vomiting. After many many ER, hospital and vet visits she seems to be ok, I am just thankful that she is still alive. However, it seems that now she is having a serious breathing problem. She will do this thing when she is laying down where she completely stops breathing for about 3-4 seconds and then quickly lifts her head and gasps for air. This is giving me serious anxiety, I watch her at all times to comfort her. I have tried googling this but cant find anything. She has had xrays and ultrasounds on almost every part of her body and her veterinarian says everything is normal. Can anyone PLEASE, please let me know what they think this might be, and/or what I can do to help her?
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