----- RESOLVED -----
Alright, so I was messing around with a sort of day/night cycle.
The basic setup:
A terrain.
An empty game object set to 1000,-250,1000
A directional light as a child of the empty. Set to 1000, 3000, 1000 (though really, it’s 0, 3000, 0 since it’s a child of the empty. In other words, parent it, then drag the light straight up.)
Added a script to the empty to rotate it. causing the light (aka, the sun) to rotate, using the empty as its pivot point. Here, have a lame ascii art example 
S
o
Where S = Directional light, — = terrain and o = empty object.
After coding a bunch and noticing strangeness, I took a step back, and went with the very basic transform.rotate, taken right out of the manual
transform.Rotate(Time.deltaTime, 0, 0);
also tried setting the Space, with the same results.
So, as I watch in the Inspector, all is going good as my sun slowly rotates around the terrain.
watching the X value update from 0 on up. When it gets to 90, it stops (and really, hitting EXACTLY 90? pfft unlikely with Time.deltaTime). The object actually keeps rotating, but the value stops updating, but only for a few seconds. But in that time, the Y and Z values change from 0 to 180 (both of them). Then the X starts up again, except now, because the y and z have changed to 180, X is now counting down towards 0 (and it continues on into 350s, right on down, as I would expect).
I think I may be going crazy 
after noticing that, I decided to take a step back even further. Set up a brand new scene with the only thing in the scene being a capsule with a script attached to it. No extra code, no extra objects. Nothing to interfere. Here is the content of the script attached to the “sunEmpty”:
function Update () {
transform.Rotate(Time.deltaTime, 0, 0);
}
Same result. Anyone have any clue as to what is going on?
fwiw: 3.5.0f5