Replacing the mobo is not too difficult. Several sites are available for the parts needed. Googling "thinkpad t42 system board" will provide everal sources.
Here is one such site:http://www.thinkpad-parts.com/39T0279-Thinkpad.html
What you are describing sounds like a heat or power problem. Have you used a can of compressed air to blow the dust out of the input and output fan ports?