Oh, I think I am seeing double. : )
I could have sworn I saw this in the Slack section too, he he he. Anyway...
When you ran xf86config and it asked you what resolutions you want to use (I think it does anyway?) did you say what you wanted? Also, try pressing CTRL ALT +/- more than once. You may eventually get to a res you like/need.
If not, post the relevant section from your XF86Config file, and see if anyone can see what's up.