Did NOT fix.... after 20 miles of mostly 60 mph cruising, I pulled back into the mechanic's shop at a hair under 250. He looked at the temp, swore a couple of times, and took the keys.

After I got it back from him last Friday, it seemed good. Ran around 210 most of the week, went to 220 Wednesday (I attributed this to the weather going from low 90's to 105), and today after about 10 miles it on the highway it was 230, 15 miles it was up to 235. Then, literally, in the time it took to complete a simple left turn it jumped from 235 to 245, and 4 blocks later at the garage, 250. Engine ran completely normal the entire time.

Seriously want to know what the hell is up with this.

 

Did you use a scan tool to see what temp is showing?

 

You probably have a clogged radiator that got that way from using the the wrong radiator flush or the mixing of a real old coolent that had stop leak in it from thr factory with the more modern stuff... In the system somewhere is a blockage and it can move around. Most likely in the radiator itself. A radiator shop or your old bastard mechanic can replace the radiator or just heve it boiled out under pressure by a shop specializing in this. New is better and AZ is no place to risk an engine boiling over.