I have the same issue.

I'm using Ubuntu 18.04.2 LTS on a Dell desktop 

Mostly when copying text, but sometimes when also opening a pdf and other applications, the system freezes for approx 30 seconds. After freezing for 3 or so times, it also logs me out. It also appears that the freezes become more frequent right before logging me out.

Before my last update, copying text would simply close the application from which I copied text. Now copying just causes a freeze about 1 in 5 times.  

Here is my gnu log:
16:17:01 cron: pam_env(cron:session): non-alphanumeric key '"' in /etc/environment', ignoring
16:16:05 spice-vdagent: Cannot access vdagent virtio channel /dev/virtio-ports/com.redhat.spice.0
16:16:00 gdm-session-wor: pam_env(gdm-password:session): non-alphanumeric key '"' in /etc/environment', ignoring
16:15:54 gnome-session-b: CRITICAL: We failed, but the fail whale is dead. Sorry....
16:14:38 sudo: pam_env(sudo:session): non-alphanumeric key '"' in /etc/environment', ignoring
16:04:43 spice-vdagent: Cannot access vdagent virtio channel /dev/virtio-ports/com.redhat.spice.0
16:04:37 gdm-session-wor: pam_env(gdm-password:session): non-alphanumeric key '"' in /etc/environment', ignoring
16:04:21 gnome-session-b: CRITICAL: We failed, but the fail whale is dead. Sorry....
15:55:38 pulseaudio: [pulseaudio] bluez5-util.c: GetManagedObjects() failed: org.freedesktop.DBus.Error.TimedOut: Failed to activate service 'org.bluez': timed out (service_start_timeout=25000ms)
15:55:37 spice-vdagent: Cannot access vdagent virtio channel /dev/virtio-ports/com.redhat.spice.0
15:55:30 gdm-session-wor: pam_env(gdm-password:session): non-alphanumeric key '"' in /etc/environment', ignoring
15:55:14 spice-vdagent: Cannot access vdagent virtio channel /dev/virtio-ports/com.redhat.spice.0
15:55:08 gdm-session-wor: pam_env(gdm-launch-environment:session): non-alphanumeric key '"' in /etc/environment', ignoring
15:55:04 gnome-session-b: Unrecoverable failure in required component org.gnome.Shell.desktop
15:55:04 gdm-session-wor: pam_env(gdm-launch-environment:session): non-alphanumeric key '"' in /etc/environment', ignoring
15:55:02 gnome-session-b: CRITICAL: We failed, but the fail whale is dead. Sorry....
15:17:01 cron: pam_env(cron:session): non-alphanumeric key '"' in /etc/environment', ignoring
10:57:03 sudo: pam_env(sudo:session): non-alphanumeric key '"' in /etc/environment', ignoring
10:31:39 pkexec: pam_env(polkit-1:session): non-alphanumeric key '"' in /etc/environment', ignoring
10:28:45 pulseaudio: [pulseaudio] bluez5-util.c: GetManagedObjects() failed: org.freedesktop.DBus.Error.NoReply: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
10:28:27 spice-vdagent: Cannot access vdagent virtio channel /dev/virtio-ports/com.redhat.spice.0
10:28:00 gdm-session-wor: pam_env(gdm-autologin:session): non-alphanumeric key '"' in /etc/environment', ignoring

and in my /var/log/syslog file here are the lines containing "Error" right after being logged out due to a freeze:

GDBus.Error:org.freedesktop.DBus.Error.NameHasNoOwner: Name "org.gnome.Mutter.DisplayConfig" does not exist
May  9 16:04:43 robert-Precision-T3600 gsd-sharing[4719]: Failed to StopUnit service: GDBus.Error:org.freedesktop.systemd1.NoSuchUnit: Unit gnome-user-share-webdav.service not loaded.
May  9 16:04:43 robert-Precision-T3600 gsd-sharing[4719]: Failed to StopUnit service: GDBus.Error:org.freedesktop.systemd1.NoSuchUnit: Unit rygel.service not loaded.
May  9 16:04:43 robert-Precision-T3600 gsd-sharing[4719]: Failed to StopUnit service: GDBus.Error:org.freedesktop.systemd1.NoSuchUnit: Unit gnome-remote-desktop.service not loaded.
May  9 16:04:44 robert-Precision-T3600 gnome-shell[4606]: Error looking up permission: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.impl.portal.PermissionStore was not provided by any .service files
May  9 16:04:47 robert-Precision-T3600 gnome-shell[4606]: [AppIndicatorSupport-WARN] while calling AboutToShow: Gio.IOErrorEnum: Method 'com.canonical.dbusmenu.AboutToShow' returned type '()', but expected '(b)'