I'm glad the setting helped out with regular text.
Yeah Mtext has it's own editor, so that setting will not help with that. You can always use regular text and not use MTEXT.
To try and track down the issues, try some of the following
I would check your video driver for any updates.
Try a lower resolution or higher resolution and see if that helps.
Verify if you are using any font substitutions that are possibly corrupted.
Check the style command and use a different font all together (use SHX font and not a True Type Font) and see if that helps at all.
if all else fails run your computer in protected mode and see how autocad acts like that, granted it will be crappy to use at that resolution, but it will give you something to discover and work from there.
Good luck