AVR crashing/freezing in the cold?

HeaterMeter checked out on me for my Christmas brisket smoke yesterday. I started setting up around 5am, which included bringing HM outside where it was around 25 degF. It had been plugged in inside for at least several days before this and was fine. A half hour later or so, I plugged it in to start controlling my fire, but all I got was 1 row of black blocks on the LCD. Power cycling resulted in various other combinations of crap on the LCD and LED's on, but no-go. The RPi was working, and LinkMeter was up but it wasn't getting any data from the AVR. I brought it inside, and after a few minutes it was functioning again. Brought it outside, and it worked for a little while but crashed again eventually as the ambient temp reading got down to around 30 or so. It actually froze up then, still displaying the temps it had last read and the fan still running but again no data to LinkMeter. One more warmup, same result, I gave up. I think the last time it crashed when I was trying to update my max fan speed parameter, and it reset some of the probe config settings (which I didn't realize until later) so at that point I just decided it was possessed.

Last summer I had some trouble with it when the sun moved on me and ended up shining on HM, which I thought at the time was actually the power supply overheating but maybe not. And then over thanksgiving I smoked a turkey when it was probably in the 30s, and the first time I plugged it in I got boxes on the LCD but after a few power cycles I got it to work. Anyway, I haven't really done any troubleshooting yet, just wondering if anyone else has had any temp issues. It could be my poor soldering, but it seems like the entire AVR is crashing and freezing, oscillator maybe?

Materials expand and contract with changing temperatures, I suspect you have some iffy solder joints on your HM board.
What I would do is remove the rPi, remove the ATMega chip from its socket, then do a shotgun reflow soldering of he board, concentrating on the ATMega, the rPi socket and power connection/regulation components. It doesn't really take very long to reflow the whole board... heat each solder joint while adding a little bit of solder, wiggle the component legs a bit with the soldering iron in the process to get the solder to flow through both sides of the board. Then reassemble and I bet your problem goes away....

...or you could start running your probes/blower/servo through the CAT5 cable so you can keep the HM inside, out of the cold.

I will try a reflow, thanks for the help!

Just as a personal experience, I have ran my Heatermeter outside when it was 18*F outside. No issues. It was just under my grill cover on my table.

I always cover my HeaterMeter with something waterproof when outside. It helps to prevent water damage and condensation issues.