Google Cache Gone - Reasons?

Reasons for removal of Cached pages

[CrackSeo]

I don't know if this topic has already been discussed in this forum, but i would like get some help from experts as this is a very serious problem i am facing. We have some sites where the Cache of the pages is gone after some time.

I would like to know what are the factors google sees and removes the cache of the page. Why is the cache removed even when the page is not over optimized.

We have 100's of pages but is optimized for several Search Phrases, and the content on these is similar (Not duplicate).

we also add 100's of pages at a time all of same size. Will adding so many pages to a site at a time efeects?

What is the maximum limit of Number of links on Sitemap. Will this also effect in REMOVAL OF CACHE?

Please guide me how to avoid this problem. Also tell me how to get back the Cached pages