One fact is established that google vanishes pages indexed time to time.But what is really serious critical level. What is your experience with vanished index pages and same level of traffic from Google or there could be some down time as well?
This was a problem several years back: http://www.markcarey.com/googleguy-says/archives/discuss-missing-index-page-in-google-results.html This may put it in perspective: http://forums.digitalpoint.com/showpost.php?p=674938&postcount=24 Are you experiencing this of late? Right now? Could you post or PM me the url? I would love to see it.