Hello i dont know why after generation sitemap, the google remove all index page & add my page every two months Do you know why ?
lol, this may be some other problem not with sitemaps, i regenrate my sitemap and ping to google every week or everytime my site get updated... and i have no problem like you are having...
Google has done the same with two of my sites, after I submitted sitemaps I got completely deindexed and lost all SE traffic, useless garbage.
Sitemap is only useful for new and unindexed websites. It will direct spider to crawl your website, and thus increase the possibility of indexing. But crawl doesn't mean indexing. And sitemap has no relationship with index or deindex.
IMHO and for me personally, sitemap gives more benefits than harm. It's more than just indexing our site, we can also know and monitor which keywords our site rank well.
I agree that it is beneficial to new sites and getting new pages indexed. You can let google know quickly about changes and new pages which is a great thing. Ive found the program to be very handy! (and its free!)
I also have an old site, already around for about 4 years, with sitemap I also got some keywords and ranks which sometime surprising me. After knowing that keywords which rank well, I know and get more clearer decision on what to do.
A long time ago, I though it was useless, but that changed last month. I had a site that wasn't cache in ages even though content was added to it daily. Also not many backlinks showed up for it in google. Well one day I added the sitemap, and now my site has a new cache often and more backlinks.
I have resorted to using a single URL sitemap, my home page, for submission. G still crawls my entire site. Why do such a thing? Because sitemaps gives you valuable info about your site, whether you provide an all inclusive sitemap or not.
I had also this problem before some days. My 7K page was ranked in google before submition but I lose all pages after submiting my sitemap
Google Sitemap still works like beta. I am not sure if they really care about sitemaps. Cause i have lots of sites with more than thousands pages. It's only indexed 1/3 of them.