My all pages are present and reachable but still google webmasters tool is showing 404 errors. How to stop it?

I am seeing some massive changes in my rankings since yesterday. Some of keywords have risen and fell terribly. :o
Has anyone noticed in yet??

hiii frienda

what is google bowling

Is it okay to publish articles on the same keyword more than one time?

Can anybody suggest me some good A/B testing tool? Any kind of information is highly appreciated.

Overview: Analyze your link profile compared to competing websites, and find pages which would be great sources to get links from. Many modern major search engines consider links to be signs of trust or votes for websites. Great in content links also can send a large stream of targeted high value traffic.

If I transfer my links from one sub-domain to another then will it hurt my rankings? I am ranking on many keywords in the first sub-domain but now I want to rank for my second sub-domain.

What is the general length of an article should be if I posting an article in the forums of my niche? In order to do forum marketing effectively.

Can I run Google Adsense and any other network such Chitika or Adversal side by side?

Has anyone came through across any search engines that is good for educational journals? Please share if any.