Female Zebra finch unable to stand or perch

Question
Several weeks ago one of my female zebra finches was having problems standing on her legs or perching. She floundered on the bottom of the cage. Her wings seemed to be working, as she can fly up to her nest (it's a large cage). I moved her to a small cage with a lamp to keep her warm and fed her boiled eggs and took her out into the direct sunlight for several hours. After a day or so she seemed better, and went back in the big cage. She is again showing these symptoms, and after a day of eating boiled eggs and getting several hours of direct sunlight (I took her outside) she seems better. Any ideas of what may be happening?

Answer
This almost sounds like a brain injury or the bird equivalent of a stroke. Has she flown into anything (like a window) recently or done anything that involved her hitting her head?
Are her eyes "sleepy" or is she alert? Is she puffed out or are her feathers still well preened?