Toshiba 42HP83 Intermittent Flickering Rectangle

My Toshiba Plasma 42HP83 is about 4 years old.  While watching tv, a dvd, or playing a game, a flickering rectangle about 2 inches wide by 6 inches high appears in the top/middle of the screen.  It started happening infrequently, but now seems to happen more often, usually after the set has been on for a few minutes.  It also goes away for a while, but comes back.  I also think it may be getting bigger.  Any advice for me?  Is it worth fixing (no warranty)?

Thanks everyone.  Happy New Year!