I have had issues with my iPhones and iPads in both cold and hot temps. In cold, it just shuts down telling you it’s too cold, and when getting hot, it becomes unstable like you observed prior to just shutting down. These things are like little solar panels, and it’s important to shade them from direct sun any way you can, even if it’s just your head creating the shade. It also helps to give the CPU as little load as possible by shutting down all background apps and going into Airplane mode after caching whatever maps you need. Normally, that will take care of up to about 90 degrees, and if you want to guarantee stability above that, you could use one of those gel ice packs under the device to cool it.