We are also having the same problem (all menus disappear except for Host menu) with a 04.05.05 site, Page, Moderate Caching, No Compression, No Whitespace Filter.
This site only has 12-20 simultaneous users, and it appears to happen if they log on around the same time.
There is also an “Application Started” in the event log around the same time as the problem occurs. It has no corresponding application shutdown.
A Restart Application clears the problem temporarily.