I'm having the same problem. My site is approximately 4 mos. old. It shows up for a few days ranking well in the serps, and then "poof" it's gone. This has gone off and on for a few months now.
Here are the characteristics: It's a .php site though the homepage is done in static html (with a php extension). It is well optimized in a similar format (no duplicate content though) to other high ranking sites I have. It has a lot of directory listings and one link from a PR8 site in Google. We implemented a 301 redirect about 40 days ago from the URL without the "www" to the URL with the "www".
Has anyone solved this issue so far? Will it stabilize with time?
I have a 6 year old content rich website which has been #2 listed for about 4 of those years. April 1 it dropped 2 positions while portal sites and Shopping.com type sites (with nothing but black hat techniques) moved up into my position.
Now my site "Ping Pongs" (yes, tennis is ok too) in and out of the web search results very few hours. In #3 position, then gone, then back, then gone...FAULT!
One site had 1,200 pages indexed in Y! but never received more than 20-30 uniques/day. Then over the last week after doing a complete css overhaul, the traffic grew 30% or so per day. The thing is that during this growth and even now you can search every couple minutes and get old (us way down) or new (us way up) results. It's definitely ping-ponging right now, but for the most part it just seems like a shift between two specific data sets.
My site was #1 for a competive term for about the last 3 years and recently dropped to #6. Everybody else just sort of moved up the ladder. It's the first time I've seen it budge since it was included in the directory.