I'm really sorry it took me this long to respond.
- Try with a clean config.
With the binary downloaded from http://quakespasm.sourceforge.net/download.htm
it happens with a clean config too.
- try the Linux binary from http://quakespasm.sourceforge.net/download.htm as well as the Ubuntu one you can install with apt. Does the issue happen on both?
Ok, I've installed Quakespasm via the Software Centre or whatever it's called nowadays (which I guess is the same as installing via the terminal with apt. It's called "quakespasm-beidl", though. Is that right, or did I accidentally install the wrong thing?). As far as I can tell, the problem doesn't happen with this version of Quakespasm, or at least it hasn't yet.
- Type "condump" and upload the resulting file
Should I do this for both versions of Quakespasm I now have on my computer? And where do I upload it?
- Do you get a reasonably good framerate? What does "host_maxfps 1000" then "scr_showfps 1" show?
Under both versions of Quakespasm, my framerate is initially just under 200 on the start map and then rises a little, while hovering between 200 and 300 on e1m1. Is that good? The version downloaded from http://quakespasm.sourceforge.net/download.htm
seems to have a slightly higher framerate, though, for what it's worth.