My machine will power up for a while, then it will start rebooting itself constantly. Some times every ten or twenty minutes while I am working. What is going on with it?

it could be over heating ,make sure it clean of dust inside the box ,and please don't come back saying i done that all ready! good luck

I agree with Caperjack's reply for what to look for first. If that doesn't help, then I would mention that a poorly power supply can cause these symptoms. If you can borrow one from a known good machine, that might be worth a try. Make sure it has at least the same power rating as yours has.
If you have recently change or added any hardware that might put additional load on your power supply, it may simply be over-loaded.
You can search www.computershopper.co.uk for more about power supplies.
Good luck!

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