I'm also having the issue of my site completely disappearing from the index. I have ~200.000 pages and ~150.000 images and all www.live.nl tells my is:
Just 10 results, yesterday it was 52, the day before it was 92.
I'm under the impression the Live search index is suffering from cancer - and it's spreading fast.
My site is clean - I'm not a big fan of SEO (Google seems to love my site no matter what I do) and the site is an authority in it's field. Why is this happening?