I have a Dell Dimension 8400 and it boot intermittently and then powers off. Is this the system board or a possible fan issue?
Could be the power supply failing, or a separate heat issue. You'll have to troubleshoot by replacing parts one by one. The board is available at a few venders or ebay. The power supply may or may not be proprietary; do some more research. Newegg had some ps that were made for dell by pc power and cooling that were around 500w; don't know if they still carry them.