My computer is rebooting alone. Please help

[Jesse Gersenson]

RAM fails sometimes. Burn the Memtest86+ ISO to a cd, boot from CD and it'll test the ram. That's a handy cd to keep around. There's also a 'Download - Auto-installer for USB Key (Win 9x/2k/xp/7)'

[MikeGL]

Is this a laptop? If not it could be a defect in the power supply, which causes a power glitch that triggers the power-on reset. I once had that problem in a VCR: I could not record on the timer, because it would always stop after 10 min or so. When the error became more frequent I could observe them on an oscilloscope. Turned out the power transistor of the volatge stabilizer had an internally broken wire, that off and on made contact or not.

Happened to my laptop too, keeps on rebooting on its own both on windows OS and on Linux. So I googled my laptop brand and similar problem was encountered by others, it was related to power supply circuit in motherboard of laptop. Turns out to be a single capacitor doing power management. With the help of Google I got the exact location and part number of that one capacitor, removing it and soldering a new one solved my problem. Cost me less than 2$ USD, and some priceless time but was worth the effort and learning experience and ofcourse 1 less laptop junk on the planet.

Going down the oscilloscope level like what you did is for über geeks. And I once thought you were only good on software programming side.

[stevenaaus]

Good work replacing the capacitor. Laptops are tough little nuggets to get into.

cpu temperature is around 5 C less that it was usual a few days ago

That is not a significant temperature change really. It may be enough to slow reboots, but is probably not a long term fix (if overheating is the cause of problem).
It's possible your RAM dimm had become loose and you have fixed it.