my presarios goes off

when i turn on the machine it refuses to boot. it just goes off, and when it boots i use it for afew minutes and its off again. when it manages to boot uptime reduces, i that if it boots i use it for 1hr, then it goes off, if it boots again i will manage to use it for 20 or even 10 minutes then off it goes, until when it refuses to boot again. making me wait then try again

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