Black Box on TV Screen?

My uncle bought a new HD television and when the commercial broadcast stations switched to digital, a strange problem has arisen. On one of the "main" network channels (ABC, I think) in the New York City area gets this translucent black "box" on the screen, that usually stays blank but sometimes displays text exchanges. It pops up; stays there a minute or so, than goes away, only to reappear a few minutes later. It seems to have something to do with propagation, and is worse on cloudy or rainy nights. Any body have any idea what might be causing it? More importantly, any suggestions on how to keep it from happening or ways to get rid of it for longer periods of time, if prevention fails?