Thanks for the replies, but please read what I wrote.

I've already put in a new power supply and it was exactly the same from a cold start the next day.

I really don't think it's overheating, as it's actually back to front for that, ie, it stays on longer each time. When a PC overhaets, it shuts down quickly when you restart it because the CPU is still hot.

I'm inclined to thing it's something on the mobo that starts to work once it reaches a certain temperature. Finding which particular component and fixing or replacing the mobo ends up not far off the price of a new PC.

Convincing them to replace the PC, given that it works all day once warmed up will be difficult. One of those jobs where you end up looking like someone who's trying to get them to part with their dosh for a new system.

They don't realise I make less money on new system than I do on repairing/upgrading an old one.

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