Based on my experiences, I'm agree with Tommy.. As you confirm, the CPU temp isn't issues, but there is as well DDR memory. It's unfortunately native that memory chip are faulty from manufacturing (since ~15years experiences, I can tell you a lot of stories), and are sensitive on temp too. And bad for us, it's difficult to test in ARM platform.
I'm just tried connect Linux RX/TX console and I'm see there isn't way to access uboot.
For example, in company what I'm working for, we have kind of memtest included in uboot. Looks like that version of uboot cannot be controlled, or?
If you interest about console, you can use windows like me, the good one console program is putty.exe, speed is working under 115200bps. Power +3.3V isn't required. But most likely you will get crash dump from kernel and you figure out, that something is wrong, what you already well know.