i suppose the load is too high at a cold boot. this starts the dievices, : disk and cd drives, fans...and loads the capacitors.
next boot has a lower load and is fine.
conclusion : you have a bad or a failing power supply
you can calculate the power you need here :
http://www.extreme.outervision.com/PSUEnginenote that older PS's deliver less than stated, due to aging
it can also be bad capacitors on the mobo (less likely) - look for bulging or leaking ones like here :
www.badcaps.net
