There are a couple of different things that could cause this problem, the first is the power supply, and the second the main board. Also either sustain or scan driver board can cause this problem.
To know exactly what board is the cause, you need to have electronic repair experience, and test equipment. If you have this let me know and I will assist you in the troubleshooting of this tv. If this is the case, then please do not rate this answer yet, just reply back with the CONTINUE CONVERSATION button and we can get started.
Otherwise you will need a repairman at this point. You can expect a bill of anywhere between $250-350 on average which is really a lot of money to put into a 42" tv these days given the prices of new sets out there.
I am sorry I don't have a quick fix or easy answer for you on this one, please let me know if you have the skills and tools and I will be happy to assist you. Also 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 remember to only rate my answer when you are 100% satisfied. IF you feel the need to rate "helped a little" or "I expected more", please stop and reply to me via the CONTINUE CONVERSATION button with the issue you have. I will be happy to continue further and do everything I can to provide you with the service you seek.