Dimension E520 Constantly Reboots...

It seems like you already have a faulty processor or its fan cannot carry the heat given by your processor. Cleaning the fan usually helps, or a much better idea is to replace it with a much larger fan.

You mention that the diagnostics load ok, but do you have any error reports if you run them?  I have had this symptom as a result of a bad hard drive.  Windows will swap data to and from the hard drive even if you have quite a bit of RAM.  if an error is produced while reading the data from the virtual RAM, a lock-up can occur.  If the problem is heat related you should see a problem similar to the one you are getting in Windows.

If you can get into the page that allows you to change the Windows settings for Startup and Recovery you should make sure the setting for Automatic Restart has been unchecked.  Unchecking that option will allow you to see any error message that might be produced by a blue screen.

The last problem I had with my own E520 was due to a failure in one of the factory delivered memory modules.  The diagnostics ran clean, just as you report, but Windows kept giving me a blue screen with a memory parity error.  Once I replaced the modules the machine ran fine.  You need to stop Windows from automatically restarting, however, to see something like this.

@JackShack, I did do diagnostics, but they all passed... That's why I have no idea what's going on with it

You might try replacing the fan. As you've changed the mainboard, you must have cleaned out any accumulated dust from the heatsink fins, so I'll presume that's not the problem.

www.ebay.com/.../380437939928

The OP says the RAM has been replaced, however, and the hard drive as well.

I have tried everything suggested.... No luck

Have you tried reinstalling the OS?

You haven't mentioned as yet if the computer reboots or freezes during an extended diagnostic test, nor if you can change the Windows start-up behavior to stop the computer on an error rather than perform an automatic restart.  If the computer freezes at some point after you have selected the option to halt on an error you might get an error message to help.  If your computer still performs a restart even after setting Windows to halt on an error, there must be something that is triggering a reset on the processor.  If that is what you have, it is almost certainly a hardware problem.

EDIT:  What add-on cards are installed?  Might you have one that generates a restart after some external stimulus, like an Ethernet card perhaps?

I've used a USB OS with the hard drive removed. So there is no way it has anything to do with the OS.

It stays running during diagnostics fine! There is no bluescreen or halt at all on windows. I removed the Ethernet card that was in it long ago