I have two installations of Ubuntu 15.10 on two different machines and on both of these machines I am occasionally seeing a lot of lag in the graphics displayed. This manifests as, for example, clicking to close a tab in Chrome and there being a lag of ~10 seconds before the tab is shown as having been closed or extreme lag when moving windows. Occasionally this lag progresses to a complete freeze of the graphics and necessitating a reboot (causing the loss of some work too).
Whenever this lagging/freezing behaviour is observed, it appears that everything else is working -- sounds keep playing etc. It also appears with much greater regularity when Google Chrome (version 48.0.2564.116) is in use, particularly when playing YouTube videos.
I should mention that this is a recent problem on both of these machines and has not been observed in either of them previously.
I don't know how to approach this problem or even how to phrase the question in a specific, technical way. I would like the problem fixed, but my question is: how should I diagnose this problem? How can I describe it in a technical way to seek assistance?
