[Q] Nexus 5 dropping signal randomly, reboot required to fix topic

My phone is dropping cellular signal without warning. Cycling airplane mode or cellular network mode does not fix it. Only rebooting the phone has succeeded in getting signal back. This problem started last night. Since then, I have experienced it three times.

Further info:

I've had the phone for a little over a year. I did a factory reset when I installed L. I am running stock.
Any idea what's wrong and how to fix it?