Problems with apache processes

Hello everybody. I´m using phpgroupware in an Apache 2.0.46 server with 
PHP 4.3.2 enabled and MySQL 4.0.13 database on a Solaris 8 operating system.

I have 20000 users using email and calendar applications deployed in my 
environment. The problem is, that after a couple of days working 
everything fine, my httpd process (apache childs) start consuming all 
the CPU of my system and causung everything to work slower and slower.
Does anybody have a similar problem? Is this a problem of the 
application or maybe the versions I´m using? Does phpgroupware works 
fine in environments similar to mine (20000 users and calendar and email 
application)?

Thanks for any suggestion

Jorge