I had a similar problem with my phone. It kept getting weak signals and would go from 4G to 3G repeatedly. Here is what I did. Power down the phone completely. Once off press the power button and the volume down button and hold it for approx. 5 seconds. It will go into hboot and start checking a file named PG05IMG.zip. Once it completes and verifies the file it will ask you if you want to update the file. Press the up key for the volume and it will update the file. Do not power off the phone while it is updating the file, just let it do it's thing. This is your radio. Leave the phone locked for a few minutes and check your signal. Once you get restarted and get 4G back unlock the phone and check it. It worked for me.
