negative without bouncing back after a few days, or always staying negative?
If you drop and stay dropped then you're doing something wrong with your link building (probably using too much of the same anchor, or only linking to the same pages without adding more content, etc).
Also, do only the linked pages drop, or you keep losing other pages, too? Because - again - if it's only the pages that you're linking to then it's definitely something wrong with your backlinks, but if other pages drop, as well... then the culprit might be other, and 3 things I can think of are...
- the content is not relevant anymore, in which case you need to update it with up-to-date information (1 or 2 short paragraphs should do it)
- your competition keeps outranking you (low probability based on the symptoms you've described, but it could happen)
- the dropped pages are orphaned (ie. they have no traffic and / or no backlinks, internal or external, and google sometimes gets rid of such pages)
So, consider either updating the content (if the dropped content is on topics that require constant updates)... or building a few links to the dropped pages that have no links and no traffic yet (preferably build links from pages that already have traffic coming into them, or which at least rank in top 10 somewhere)...