Is she biting or chasing her tail in circles. If chasing read here:
Tail Chasing in Dogs
If she is biting it, eactly where on his tail is he biting. The base, the tip?
Also has the vet checked his Anal glands?.
Also you say she will do it every 45 seconds. When she does, how long does she do it before she stops?
Biting his tail can also be a compulsive disorder and also can be caused by neurological disorder. Or simple attention getting behavior, boredom, restrictive play, etc
Carol and Jemma