ok there must something wrong either in the power supply or in the main board,
since the tv works for seconds and then goes off, this means that the tv goes to protection mode due to some bad components either in inverter board or main board or t-con board, what you have to do is isolate the problem, try removing the cable that goes to the inverter board and turn on the set, if the tv does'nt go to stand-by again this indicates that the inverter board is defective, do the same thing with t-con board by removing the lvds cable that goes from main board to t-con?
if this does'nt lead to any result then suspect a defective main board...
try touching on regulator ics if they get very hot on main board, high esr capacitors either in psu or main board could cause similar problem...
by doing this it must lead you to some conclusion.
good luck