A minute after log in the tpanel becomes unresponsive

[Teunis]

Two days ago I started noticing some slowdown of the system, it would happen after many hours of use.
Since late last night the panel with taskbar and all what is on it starts as it should but after a minute or so it becomes unresponsive.
Right clicking on the desktop gives no response.
I can still start up some other apps like Firefox, Pan and Thunderbird as the assigned hotkeys are still working, the exeption is Dolphin, it won't start up at all.
Shutdown is only possible from a terminal or Konsole.

I wanted to look what single item was updated two days ago but Muon's History ilooksempty and will lock up.

I reverted from the nVidia driver to Nouveau but no dice, in Nouveau I don't even get a panel.
From the CLI I reinstalled the nVidia driver, rebooted and set up a new user.
Rebooted and logged in as the new user and within ~20 sec. the panel was locked...
As there were no available hotkeys in this user I could only shut down via the terminal.

Unless someone has an idea I'm getting close to a reinstall...
In the mean time I'm using the parallel booting 14.10 that works fine.

[Teunis]

It's (for me) too early to say the problem has been solved but the system is now working as expected.

Over an hour ago I used the 'nuclear' option by deleting .config, .cache, gnome, .gconf, .kde and .local.
Rebooting restored them, sure I lost personalisations, but to prove how solid the underlying systems are Kubuntu recovered it's Plasma desktop.

Yes it would have been interesting to do them one-by-one but I didn't have the time nor the patience

[Teunis]

After the above 'repair' I was running on Nouveau.
I wanted to give nVidia another chance, this time via the edgers ppa and installed the latest version 352.
It worked better than 340 and 346, it took some 10 minutes before everything became unresponsive.
After watching the screen for another five minutes it was again possible to click on icons and get a reaction.
After the power saver turned off the screen recovery was only possible via the Magic Keys.

I'm back on Nouveau.