It is sounding like a hardware problem to me, more and more. A bit more information:
1. It is ALWAYS in 4G mode when it freezes. And ONLY the mobile data freezes, the phone never locks up. Even when mobile data is frozen, I can turn on WiFi where available and WiFi works.
2. As Drew5150 suggested, the last time it happened, I switched to "CDMA Only" for a minute, and it worked on 3G. Then, at least one time, I switched it back to "LTE & CDMA", and after a few seconds 4G came back!
3. Still seems to happen in most areas, even where there is a fairly strong 4G signal, like 5-10Mbit. But twice when I pulled up near the Sprint store, which as a 20mbit 4G signal, it came on. Like that really strong signal overrode whatever was causing the problem.
I'll watch it a few more days. It's under 1 year warranty, so I've got 10 months to take it back if I need to.
Thanks everyone, this was a lot of help!