Loss of picture on Pioneer Plasma Tv

We have a Pioneer Plasma TV PDP 620FD which we use mainly for movies. When viewing TV, we use amplified rabbit ears. Recently, when viewing broadcast TV in digital mode, we began experiencing interruptions in picture. Sometimes the Pioneer logo comes up. We press the enter button on the remote and picture is restored. Other times we lose the picture entirely and can't re establish it. If we leave the TV off for a few hours, we are able to get picture again. It plays well for about an hour, then loses the picture again. We have browsed for information but we have not found anything that addresses this problem. Initially, even when the broadcast picture was lost, we could still use the TV as a game or movie monitor. Now it seems we can't get any picture on it.