Brian, since it is working fine with an external monitor, I seriously doubt it is the motherboard. I would probably start with a new inverter, then an LCD. Also, check the cable, it is possible that it does have a short in it, which could cause that behaviour. Something [http://www.itsurplusliquidators.com/ibm-lenovo-thinkpad-t400-sub-card-inverter-42w8230-42w8229.html|like this] should work and it's not to expensive. Hope this helps, good luck.