I noticed this bug starting around 14.04 LTS. It has been in every LTS release since then. I run two Kubuntu machines and they both suffer from this bug. Both machines use Intel Core technology and have 8Gb of RAM. One machine (16.04.5 LTS) uses Intel video and the other uses Nvidia (18.04.1 LTS). I have 2 monitors on the Intel machine and three on the Nvidia based. It happens more often on the Nvidia based system and I have tried various driver versions both proprietary and open. The problem usually does not show up until the machine as been running for a couple of days or so. Yes, these machine run 24/7 rebooting only for security updates.
The problem is somewhat hard to describe. An application window will freeze or stutter. For example Gkrellm, a system monitor, will show a system time of 11:05:22 then bounce back to 11:05:20. It will toggle between these rapidly until I click on the window or restart the app. Clicking on the window will unfreeze it for only a few minutes however. I believe the applications are still running and it's just the display that is effected. Eventually the whole desktop including Plasma becomes effected and a reboot is necessary.
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