50 in plasma tv black square apears when turned on then fades after an hour?

when i first turn my 50 in TINY tv a black box appears in the center of screen about the size of a vhs video cassette its not the same size as any thing in menu etc for it to be sceen burn and it fades from top to bottom after an hour ish ,also it seems like a contrast problem also ,because any thing white on screen sometimes flashes red, its nearly 5 years old from a company called tiny but they have since gone bust ,is it time for the bin?