cls_wired - 10:14 am on Sep 20, 2010 (gmt 0)
Tedster, i'll try to give several versions in assumption that this is Google bug.
1) Hardware or network problems on Google side. The server (or bunch of servers), hosting the part of index file with problem site in it, may to be in troubles with network latency and information can not get to ranking server in time. Due to periodical properties of network (route tables updated by cron, etc), we can see this periodical problems.
2) Or, may be, it is consequences of joint work of some inner Google programs. For instants, when site is in top, and gets a lot of traffic, one of antispam programs can find strange users activity, and pessimize the site, than, the next programm, which looks on other site parametrs, can unban it. And loop is done
Certainly it only assumptions, but knowing a bit how search engines do work from inside, I can tell that if it is a bug, the reason should be very similar.