I have the same problem with my 42" LG.  From what I have read online, the problem is with the YSUS (or Y sustain) board.  I have also read that the ZSUS and YSUS boards must be changed at the same time, but there has not been a consensus on that.  The boards vary in cost.  My YSUS costs $106.  I have found this website to be comprehensive and well priced:  http://www.partstore.com/
Good luck.
                
 Kim
                
 August 2008