In addition to frankbell's suggestion, can you confirm the battery is fully charged. Are you running it on battery or mains? A low battery charge would cause shutdowns if it was the only power source.
15 seconds is probably too short a period of time for an overtemp shutdown. I'm thinking blocked air vents, dodgy fan, but you could check that.
Is anything being displayed when it shuts down? For that matter, does it shut down properly or just die?
As jlinkels, having taken quite a few laptops to bits, I've never been able to tell they were "working" from doing this and looking at the various chips, caps, connectors, etc. What do you mean?
Give us some more info to go on and welcome to LQ!