CentOS - May 2018 - Various applications freeze regularly since CentOS 7.5 update

Hi,

since we have upgraded to CentOS 7.5 (from CentOS 7.4), we experience 
frequent, but not reproducible, freezes of various applications: 
Thunderbird, Firefox, LibreOffice.  Usually the application process 
keeps running with PPID 1 w/o being killable (not even with SIGKILL). 
Freezes of Thunderbird are usually followed by Firefox crashing a few 
minutes later.  The only possible solution we found is a complete 
workstation reboot :-(.

Other problems are that many Gnome extensions are not working any more 
and also cannot be installed from extensions.gnome.org any more and 
logging in via gdm (LDAP, homedirectories on NFS share) is considerably 
slower than with 7.4.

I don't know if these issues are related, if they are symptoms of a more 
global problem and I am at a loss on how to debug them, as they appear 
from time to time but not not reproducibly..

Any hints or reports of own experiences are highly appreciated.

Cheers
frank