My dog is having the same problem. How old is your dog? I'm a 16 year old and my dog is 15. My family came to our senses and realized that we are being selfish, and we are going to have to have him put to sleep. He is my brother and my best friend, and I have never lived in a house without him. This is the hardest day of my life. I hope that your situation is different.
I wouldn't really know why your dog falls over; when exactly does he/she do it? When he/she get's scared? Sick? Hurt? Or what? If your dog is doing this for no reason or show no signs of anything else I would call your vet and ask what you should do.