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:
From what I can see here, it looks like we haven't been able to reach your site for two weeks. Temporary downtime is something we're certainly aware of, and which our algorithms can work around -- downtime for two weeks however is almost certainly something that will have some short-term, visible effects. Over time, as we're able to recrawl the pages that failed (keep in mind that this needs to be done on a page-by-page basis), this should settle back down again.
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.
Forum discussion at Google Webmaster Help.