Mitsu vs-60607 when cold has black horizontal line all over screen. screen gets kind of "encircled" by black then gets crazy jittery then looks fairly normal. once warmed up, it is fairly normal except for a few black horizontal lines that come and go, usually towards top and bottom of screen at same time.
Sounds like a capacitance or power supply issue. If you are handy then open up the back (while it is off of course) and look for bad caps to replace. If it is not that then your controller chip is dying and there is no easy way to replace those as they are BGA, so it would mean that you have to either deal with it or get a new TV.