4.11 Server Locks Up

[Tom Grove]

Over the past few months I have noticed that our mail server is flat out 
locking up.  I monitor it via Nagios and about once every two months I 
get emails saying it is down and when I go over to the console the 
server is totally unresponsive.  I've gone through logs every time and 
find nothing at all wrong. 

This is a Dell PowerEdge 2850 with Dual Xeon cpu's and 2GB of memory.  
Uname replies with:

FreeBSD colossus 4.11-RELEASE-p13 FreeBSD 4.11-RELEASE-p13 #0: Fri Oct 
14 13:34:01 EDT 2005     root at colossus:/usr/obj/usr/src/sys/COLOSSUS  i386

One, has anyone else had similar problems with boxes just becoming 
unresponsive under high load?  Two, is there any reason this would occur?

-Tom Grove