Akai lct42z6tm. my akai 42 inch flat screen has 4 scrambled squares (1 in each corner of screen)

Question

my akai 42 inch flat screen has 4 scrambled squares (1 in each corner of screen) when i try to use a hdmi channel.If i cut the unit off and switch to a non hd channel all is fine.Funny thing is it worked fine for a while. I have a cox hdmi converter box with a phillips 12 foot cable going to tv. Should i try dvi/hdmi rather than hdmi/hdmi cable? Thanks