Why My URL’s Google Index Decrease?

I’m notice that my blog Google indexing decrease per week. Before this blog has 1600 index but now only have 1200 index. I’m not sure why this happen and still looking for the reason. This happen after the blog was UP and running again after down in 2 days.

As per uptime report by host tracker, this blog has been drop to 96% and now start to counting again to reach the normal level in 99%. But I still have problem on Google indexing. How about the Yahoo indexing?, surely I’m not notice like Google indexing. Because based on my Histats status, mostly my visitor come through Google search engine.

This is my big problem for now. All setting same like before the Blog down. I just updating the blog with new content to get more Google index but my wish does not come true because my Google indexing decrease gradually.

Do you have any idea to solve my problem?, or do you experience like this with your site or blog. For your information, my blog has been running up 3 years and using WordPress as the engine.