If I were you, I will either
a) replace the RAM with a new/reliable one (e.g. Kingston RAM), and re-test.
[Segmentation fault are usually caused by bad RAM. A simple RAM check may not be able to reveal the fault];
or --- if you are 100% sure that there is no hardward fault, then
b) backup all your data, reinstall the Linux STRICTLY following the perfect server setup guide and then reinstall the ISPconfig -- latest version. Then
--- do not restore your data yet, try to create a few new sites, see whether the httpd, ftp, email, webmail are all fine, if so (of course it should be unless there is hardware problem) , then
--- restore your backup data, test whether the problem is gone.