Your symptom does indeed sound like Uhthoff's, Karilann. I've had that, and ON. Very different symptoms (for me). Uhthoff's used to affect me in the southern heat, when I lived in Mississippi and Louisiana. For me, if was like a cloud had covered the sun on a clear sunny day. Went away after I cooled off.

My ON took away vision from the line of sight down. Everything below what I was looking at was gone. Very specific, too. I could only see the top half of words I read. That happened twice, long before I sought a diagnosis. Lasted several weeks, if I remember correctly.