This happens to me all the time in downtown San Francisco. Same thing would happen on my evo lte. I have to switch to cdma only all the time. Switching this usually switches mobile newtwork on and off. And then 3g works fine. I sometimes feel like it's also the amount of traffic in the area. As sometimes it will freeze yet later in the same spot 4g will work fine.
Sprint service no so great. I'm giving them a little over a year to change my mind otherwise I'm switching carriers.
HTC EVO 4G LTE
Hboot 1.12.222 S-off
User name autogeek on themikmik