TV?

I have A Philips TV (10 years old!) but it works fine. However, during viewing, "Greek-looking" symbols accumulate mainly, but not exclusively, in the subtitle area of the screen. Most of these symbols are 'thetas'. It gets so bad that I can't read the subtitles and I have to switch the TV off and on again to remove the symbols. The cable operator does not know what I am talking about though I suspect it is something to do with them.