Any solution to Partialy Indexed pages?

Crawled then dropped from index.

[molsen]

A site I work on has close to a million pages due to it's forum based content. Google's index reflects just a portion of them and most of the pages Google does have are partialy indexed (URL only in SERP, no description).

For each of the partially index pages, the googlebot has crawled the page numerous times. The age of the page does not seem to determine the chance of it being partial, fully, or even not index at all. The number of googlebot crawls also seems to have no bearing on the page being full or partial.

Google has also been slowly trimming out some of the partially indexed pages (making them not indexed, the URL does not show up in a search) without even trying to re-index the page first. At the same time some of the partially index pages are now showing up as fully index, also with no new bot activity.

So far we have not been able to come up with any consistencay for how, why, and when Google drops a page from full to partial index or dropping a page out of it's index. Any one else have any experience with large sites and partial indexes?