A Google Webmaster Help thread has one webmaster complaining his site is slowly dropping out of the Google index and wanted to know why.

JohnMu from Google explained that one of the reasons is because his site is simply often offline. John explained:

Yea, down here and there for a minute or two on occasion won’t have a serious impact on your rankings. But down for two weeks at a time can be devastating.

We know Google will check back a few times if a site goes offline. There are also steps you can take when you are having planned down time.

Forum discussion at Google Webmaster Help.