I'm experiencing the same problem.
On late evening Sunday June 29, my index page recuperated its top ten position. However, early on July 4 with the new fresh tags July 2 or July 3, my index page vanished again.
I just checked my main keyword phrase using allinanchor word_1 word_2 word_3 word_4 and my index page is number 10. The same index page is hard to find under allintitle or allintext.
On June 28, I adjusted the text on my index page. I brought down keyword density for word_1 and word_2 to 5.23% each and word_3 and word_4 to 4.65% each.
I checked total percentages on other 6 web sites within the top ten (word_1 word_2 word_3 word_4) and these are the results:
Web site A: 17.68% (site uses doorways)
Web site B: 7.31% (site uses link farm)
Web site C: 19.16%
Web site D: 9.65%
Web site E: 17.66%
Web site F: 18.44%
My site: 19.76%
The first six sites are still showing in the top ten under a regular search for word_1 word_2 word_3 word_4. My web site isn't showing on the top 100.
The main difference in between my web site and the first 6 is that mine is 7 or 8 months old. The first six are 2 to 5 years old.
I believe the problem is described on paragraph no 5 in a study made in late 2002 by Microsoft and HP: "The Google search engine attempts to maintain a fresh index by crawling over 3 billion pages once a month , with more frequent crawls of hand-selected sites that are known to change more often. In addition, it offers access to cached copies of pages, to obviate problems arising some of the crawled URLs being out-of-date or having disappeared entirely." [research.microsoft.com...]