James's 99 Miata

[BLINGMW]

Problem found Remove that ish! By the looks of it that is probably blocking over 50% of your airflow to the radiator!

2nd that. Mine was doing almost the same thing, though I managed to keep it from boiling. Combo of that and license plate was too much, and it was only like over 60, something about the airflow must change then. And the fan shouldn't have to run on highway. If I take off the plate or the mesh (and mine was less obtrusive than yours appears to be), it's fine.
For what it's worth, I found turning the AC on in mine, which kicked on the 2nd fan, would also solve the issue, but screw that.