I will be happy to assist you with this question.
The issue you describe is consistent with a power supply board issue.
This is the board where the AC goes to.
I do not think it needs replacement,rather some defective capacitors are causing this issue.
Open unit up and take a look at the power supply board.You will probably see one or more electrolytic filter capacitors that are bulged up in the top-not perfectly flat like the others.
The value of these capacitors will be different,most likely they will be somewhere around-820 to 2200uF/25 V( or /10V)
You can replace them with same values but I would recommend the voltage rating to be higher(like 35V for an original 25V) for reliability purposes.
You can buy these capacitors at any Fry's or Radioshack store or you can order them online at vendors like "mcmelectronics.com"(look for electrolytic capacitors).
After replacing these capacitors most likely the normal operation will be restored.
Attention!-before servicing unit,please leave the unit unplugged for aboutXXXXXall the capacitors will discharge and there in no more danger for electrocuting.
Now,I do not know if you plan on repairing this issue yourself,you will need some electronic background and some soldering equipment.
If not,then I estimate the charge for this kind of repair to be somewhere between $130-$150.
there are other possible causes for the same symptom,like a defective Tcon board or a defective inverter board,but the capacitors in the power supply are the most common one.
I hope this helps,
Thank you,and have a great weekend!