booting problem

There is an intermittent fault on booting the computer into win xp.The first screen/bios info either does not come up immediatly or keeps coming up every few seconds. Sometimes the switch off would not work so I have to resort to unplug from the mains, and sometimes it works normally. I suspect either the motherboard or the power supply. Any ideas please!!

Does it make any difference whether the PC is 'cold' or has been switch on for a while and 'warmed up'?

I think it is more likely that it would start from cold than from when it is warm.