It sounds to me like you have a convergence problem. Most often when it changes after warming up it is still at the point where no parts may have to be replaced. However you must not use it any more until repaired. The way to fix the set is to unplug it and remove rear cover. Find the large metal heatsink(s) that have convergence ic's mounted to them. Number on ic's usually starts with STK. Then you must get to where the pins of these chips come through the board. Use caution when handling board and mark plugs that have to be unplugged. Anyhow, on the underside of board just resolder all pins of convergence ic's and reassemble set. It's really not that difficult to do but you have to have solder and a soldering iron. Most repair shops will charge 100.00-300.00 to perform this repair so with a few supplies you can save alot. Good luck, Russmann.