40% pages from a site are missing from the index

but they have PR and backlinks....

[Imaster]

I am noticing a peculiar problem since a few days. A 7 PR site which was fully crawled in Google is now missing around 40% of its pages from the index. Previously, all the pages were indexed in the database and it had been that way since almost a year. The content on all the pages is unique and there is no question of spam/etc.

However, all the missing pages have good PR assigned to them and also show their backlinks, but there is no cache available. Half of these missing pages are in the database but they have only the url in its title and no other information. The rest half are completely not there.

Suggestions anyone!