Login screen frozen after screenlock

Hi all,

three weeks ago I purchased HP Z600 Workstations and installed a recent RHEL 7.2.
The users complain that sometimes the greeter-screen after the screen locker does not accept any input for a few minutes. The input screen is frozen until it gets up by itself after 60-90 seconds.
I suspected NIS / yp to be down or delayed but it seems to be OK. Syslog, KDE/GDM log files and dmesg do not show any errors or strange messages.
Any ideas on this are highly appreciated.

Thank you in advance
Stephan