Why does a raisin dropped in a glass of fresh champagne bounce up and down continuously?

It bouces up and down continuously from bottom to top. Why does it do this? Please back your answer up with hard evidence, possibly a website explaining the phenomenon. If it is the bubbles pushing the grape UP, why don’t the bubbles KEEP it there? Why does it come back down again? Why doesnt it just sit somewhere in the middle of the glass?