you have to try a bigger test then that, try this test http://www.testmy.net/tools/test/d_load.php?step1_timed=&step1_size=&step1_validation=&testall=&s=5983&st=st&ni=ni&ta=&top=&tt=1&nobar=
this could be one of many possibilities, there could be air in your tv which means that plasma gas is leaking out of your tv. it could be that the device that forms the picture is bad. it could also be that the tv is just out of wack and needs a tune up. I would just give zenith a call.
edit: spelling -D