Hi,
Aaaah Dell,s, I as a matter of course don't use DELL's pre-installs, they are notoriously flakey, I just drop the partition and do a full reinstall, it saves a lot of hair loss.
Also check for BIOS updates & download and install the lastest Video & NIC drivers. Also check for mouse driver updates, I have, as well as others, spent months identifying buggy mouse drivers. Have had problems with MS Intellimouse & Logitech.

Just reinstall & check one problematic workstation & see what happens, keep in touch.

Regards