Puppy loses use of back legs

Question
I have a two month old Chow Chow puppy.  She often loses control of her back legs.  When she is lying down and tries to get up, sometimes her front legs do not work either.  She can be in the middle of running, and her back legs just go flat out behind her, or they move stiffly and she cannot control where she is going.  When she is startled, she often just collapses on her side because she cannot move her hind legs.  I have taken her to two vets and neither has an answer.   On vet did the test whereby he pulled her back legs out from underneath her.  He said because she "righted"  her legs, he didn't see a problem.   Please help!

Answer
I would suggest seeing if there is a neurological or orthopedic specialist in your area.  If x-rays have not yet been done, this might be prudent to see if there is a spinal abnormality that may be causing trouble with the spinal cord.

Given her young age if there is some problem it is likely developmental.  Since she does still have some function of the limbs it is worth seeing if there is a structural abnormality that may be repaired surgically

I wish you the very best and hope that a treatment plan can be found.