Ok, if there are no problems with the picture when it is working then I am going to say that you most likely have a bad Z801 module...
This is a module that has been notorious for failing in your model causing it to do exactly what yours is doing...works when it wants to shuts off when it doesn't, causes weird effects in the picture and so on..
This can be replaced by a trained technician and will run you somewhere around $300-$350 for the repair. I am about 90% certain that this is your issue.
If this isn't your issue then most likely it will be a failure in the convergence circuit, this repair will run more around $400 to have completed by a tech.
There are a couple of other issues that I can think of that could possibly cause this as well but it would be unlikely, they include the vertical and the power supply, these would still run you somewhere between $300-$400.
So the botXXXXX XXXXXne here is you should be expecting a bill of somewhere between $300-$400, but the set is very repairable, you will just need to decide if it is worth that kind of money to you or not. Please let me know if there is anything else I can do for you.
If you would like any additional information or have more questions please don't hesitate to ask!
Please never accept an answer until you are 100% satisfied and feel you can leave positive feedback. If I have answered your questions and met your needs then please press the green "accept" button. Otherwise please feel free to ask anything else to get your problem resolved. Positive feedback is always appreciated!