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.
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.
Yeah, I always have this problem, when when I was on my Droid X. It will show 4G or 3G and when I go to browse something, it won't do a damn thing. Verizon/HTC needs to get their ish together