What is the reason of that ? Maybe it is the consequence of decreasing backlinks ? Anybody having this problem ?
Indexes will drop if sites have been blacklisted for breaking Google's rules among other things. I suggest you try and be concerned with quality over quantity. That could be another reason. ie Google is more worried with quality over quantity.
I've noticed the issue of index decreasing, mainly because of the type of website I have. If it's something that's unique content driven you shouldn't have that issue.