anyone know what actually causes this? "FATAL! Server is too busy to process requests"

[Tony Redstone]

we occasionally get these errors in our logs:
Thu May 18 09:31:05 2006 : Error: FATAL!  Server is too busy to process requests

and the server dies.  I've found the core in src/main/threads.c that
spits out this message but it's not clear to me under what
circumstances this would/should happen.  It happens on several boxes
ranging from redhat 8.0 to fc4, some processes are talking to backend
SQL databases some aren't (in fact, it's our plain radius
passthrough/routing proxy servers with file only based config that
seem to experience this issue the most).

does anyone know what really causes this and how to fix it?   we're
serving around 50k DSL users with a nominal authentication load of
around 200-300 requests per minute (this is for authentication only,
accounting is handled by separate processes which don't experience the
same problem).  peak load can run into several thousands per minute
when recovering from planned maintenance on part of the DSL network.

Tony