After experiencing this problem:
I rolled back to the nouveau driver. Initially it did not work, but suddenly I realised it was due to the "nomodeset" parameter in grub2 while booting. I removed that, and now I can use my computer at 1600x1200.
The question is... I tried to configure grub2 but there is no way, instead of saying the lubuntu splash I see nothing (though it boots okay). Now I set it to textual boot, so at least I know it is working.
How can I solve that?