Certain tasks make the screen go black with a blinking underscore, then I have to log in again

Trisquel on an old ThinkPad. Sometimes it crashes unexpectedly, but I can reliably produce crashes by –

* Opening GIMP
* Opening some PDFs and videos. Others are fine.
* Opening some websites. Most are fine.

Having a lot of tabs open seem to make crashes more likely. Heat seems to make crashes more likely.

Every crash produces a file in `/` called something like `core.1428141`; these are about 150MB of text each.

I think lightdm or xOrg are crashing. It's not kernel panic, I don't need a full reset, just to log in again.

Output of `pstree`: https://pastebin.com/idXfAgN0