Make sure your radio version is 6.35.07.29 by entering FASTBOOT mode. (Back + Power). If it's not 6.35.07.29, don't continue.
Download this update and save to your SD as update.zip
Boot your phone into HTC recovery mode using Home and Power.
Press Home and End to bring up the recovery console.
Choose the option to "Apply sdcard:update.zip"
If this is successful, it will apply the update then bring you back into the recovery mode. Turn off your phone, then boot into FASTBOOT using Back and Power. See if your HBOOT version is now 1.76.2007.
We might be on to something here...