I'd try the wiping the cache and reflashing the radio.
I had a similar issue first week of owning the phone any time I sent a text I lost service until I rebooted.

From what I've read some N5's don't like the 4.4.4 radios. Mine likes 4.4.4 but couldn't pick up a signal if it had a 50ft antenna with 4.4.3 radio.

Sent from my Nexus 5 using XDA Free mobile app