I had this problem with a TiBook. It was the ATI graphics chip. Apply pressure on it, and the lockup problem went away. I reflowed it twice with a heatgun. The issue got better, but still locked up if you slightly twisted the casing.

I eventually replaced the motherboard with another one I found.

However, a reflow would not hurt, and if it is not working now there is not much to loose. Monitor the heat with an IR thermometer if you can, and shield of parts you do not want to heat with aluminum foil. Pay attention to plastic parts, and the cable coming from the modem port.

Expose only the ATI chip, and gradually heat it.