It might be worthwhile purchasing a Digital multimeter and checking out the supply rails to the mainboard - Between black and yellow it should be no less than 11v and between black and red it should be no less than 4.5v. If theres any other colour wires, let us know.
Also, although the RTC battery shouldn't cause the problem since it doesn't power a BIOS like in PC's, it might be worthwhile checking that it hasn't gone below 2.7v.
