Many, many things could cause that behavior.

First, carefully inspect the mobo for telltale signs of capacitor leakage or other crud.

Recheck the power supply connections to the mobo. Sometimes, simply unplugging and re-plugging the connector can clean off oxidation that was causing flaky behavior.

A weak PRAM battery can cause mischief in some models. Try simply removing it and see what happens.

Those will do for a start.