Google is dropping sites! Old news. And very hard to know why.
Therefore I have been dropping all possible things that Google might take exception to - now putting clients on a different server, only one way links to clients (they need the PR), and text mention of parent site, not link from clients, non recip links from each page to authority sites for that page.
Have yet to get the sites back properly, but you keep trying. Once they are back, I will probably add proper links from them back to parent.
All the normal onpage seo, hopefully not excessively done.
I know it is overkill, but somewhere is the key.
I have seen one of my parent pages with same title keywords as client, kill the clients page when on same server... Interesting, my PR6 site virtually untouched through all Google convulsions. Two or three pages effected in each generally come back in the next Google update.