When we first got this TV we could receive HD channels. Now they won't stay or come in. We have scanned for channels to make sure that they are there, however, after the scanning when we go back they either fade in and out or won't come in at all. I spoke with our cable provider and they said that the internal antenna on the TV should pick-up the HD channels with out a converter. I have the TV set to Cable. Is there anything else I need to do?