Marketo Form Format Keeps Changing on Webpage

The same form, with the same styles, at the same resolution, will always look the same.

Since Marketo form styles themselves are embedded within the form (unless deliberately packaged separately) it's extraordinarily unlikely that the form (a) displays but (b) has different styles, based on any problem on the Marketo side.

What I'd suspect is instead that some CSS elsewhere in your page is not loading reliably.

Of course without a URL it's impossible to look into.