A common cause of low motherboard voltages, is a faulty or damaged 20 (or 24 pin) molex connector. Disconnect this and thoroughly clean the 2 halves of the connector - the PSU part and the Mobo part. If necessary, use a small piece of emery paper to clean any corrosion off, and possibly bend the pins slightly so that they get good contact.
I've seen a number of unstable computers where the connection on the main PSU connector has been so bad, that the connector has actually started to melt - once this happens it's a downward spiral, the overheated connector corrodes more and the problem gets worse and worse.