Content and header width changes randomly.

Hi!

I’ve been experiencing some problems with my site.
It seems to switch width randomly, I’ve attached images.
The headings will become all uppercase and then return to normal, it seems random. I’ve tried adding custom CSS and/or selecting “None” for Text Transform for all headings 1-6 in Advanced Styling with no effect.
I have a custom CSS class for 4 buttons and I’ve added Custom CSS

@media only screen and (min-width: 990px) {
.only-mo { display: none !important; }}

so these button only show on mobile. This stops working and then resumes functioning seemingly at random.
I’ve suspected a plugin, but I deactivated all of them and saw no change. Also, the symbol indicating a plugin update is available is always there, but no updates are available.
Lastly, the contact form doesn’t work, no emails reach my inbox. I think this is a problem caused by setting up SMTP.