Every day I check my status on the SE's and find that Google is the most mysterious. One day I have 200k pages indexed. The next day 55K. Then inching back up only to fall over again.

What is the deal with the erratic index numbers? Is there a clue?

I think at the moment is because they're building a new datacenter and are probably reverting back to old servers or something along the lines of that.

There's been a few articles on Google about it messing up the index and sometimes not showing a site at all.

Google does this, but I've found that they've been pretty stable with my sites. The ones they've index that bring in a little traffic are pretty consistent.

I monitor a lot of keywords and I notice the same thing as well.