I believe that the TV is pulling too much current probably due to a fault in it's CCFL (Cold Cathode Fluorescent Tube) inverter.
The power supply has built-in circuitry that protects itself by lowering it's output voltage when it tries to provide excessive current. This keeps it from overheating when a power fault occurs.
It is unlikely that the DC Power Jack is the problem. Generally, the 3 contacts on a DC power jack signify:
1) Power
2) Ground
3) NC (Normally Closed) switch contact
You can look at this datasheet to see how the 3 leads work.
**broken link removed**
First, I would do a careful inspection of all components on both circuit boards looking for any burnt areas or capacitors with leaking or bulging tops. Make sure the circuit boards are not inadvertently touching the metal backplate.
Second, I would disconnect the inverter board's connector (the inverter drives the CCFL backlights and is the board on the left side). Plug in the power supply and take a voltage reading. If the voltage is near 12V then I suspect at least one of the inverter's Power Mosfets have shorted.
creakndale