99% sure your computer is overheating - fits with the getting worse over time, and prerendered frames wouldn't make sense because most of the time that setting doesn't even affect latency (and it doesn't kick in gradually).
1 - if your computer is 1+ years old and you've never opened it up and cleaned out the dust - do it. it doesn't take that long and it can make a world of difference.
2 - it seems like you've got this down but make sure you don't have any battery-saving features enabled (sometimes these are hard to find), and check that there aren't any resource-hogging processes running while you play. also try running fullscreen if you don't already.
3 - capping your framerate will probably stop the overheating issue, but obviously you don't want to cap it too low, so it's a bit of a compromise
4 - a laptop cooling pad should help, but costs money
that's about all you can effectively do, other than upgrading your computer