When we first received our HTC Thunderbolt review unit, we -- and others, apparently -- experienced problems with 3G data not always functioning in areas without 4G coverage. Things improved after we performed a factory data reset, but that's pretty inconvenient. Fortunately, some clever folks over at Android Central have come up with an explanation and potential fix for this issue. It looks like some devices are being activated in areas with eHPRD coverage but then used in areas without. eHPRD allows simultaneous voice and data on EV-DO networks, and provides faster LTE switching -- assuming 4G is switched on. Supposedly, this feature is down in some parts of the country, so phones configured to use eHPRD can't initiate a data connection over 3G, but ones provisioned for regular EV-DO are connecting fine. One way to reconfigure the Thunderbolt is to re-activate it on the network, which is one of the side effects of doing a factory data reset. A better way is to invoke the programming menu and re-provision the device manually by following the steps outlined in the source link and illustrated in our gallery below. Sadly, we can't verify this solution because we currently have both eHPRD and LTE coverage in our neck of the woods, but some people are reporting improvements. Let us know in the comments how it works out for you and be careful not to mess with any of the other settings. You have been warned.
Update: Dial ##778# and tap the Call button to enter the programming menu. The password for the Edit Mode menu is 000000. Follow the source link for more info.