It is a known issue. I live in rural TN (far from any 4G access) and I've had this off & on since I got the device on launch day. This issue led to me returning the phone while I still could. I'm watching for a solid solution before I go Thunderbolting again.
Every so often i get disconnected from the 4g internet and i cannot connect to anything / go to any sites for a few minutes, or until after i restart.
Is this normal? I show most of the bars.
I believe the OTA update coming, which has already leaked, has fixed this for the majority of users. People who are in 3G areas close to 4G areas are still losing data though, lock the phone to 3G in the phone info menu and you'll maintain your data.