I maintain several boxes. They are intended for shared hosting and dedicated hosting as well. Some times servers hang up without any visible reasons. I've noticed, that this unexpected hangings affects only dedicated servers with large number of threads in memory (more than 30 simulteneously). Shared servers never hangs up even they have 30 or more threads of Apache. If I decrease number of threads (by reconfiguring or using web-frontend) - troubles disappear.

Does somebody know what may cause such behaviour like this? I use Apache 2. Shared servers have more than 20 virtual hosts, dedicated - 2 or 3 as a rule. In other words, I want to know, can Apache cause this hangings and how it depends on number of threads and virtual hosts.

Any ideas?