Hi there,
constant reboots may not be caused by just the CPU or Mobo having faults but can be caused by many other factors. As you have had the Mobo tested and the CPU I think it is unlikely that they are the real cause of the problem. Here are a few other things that could cause it to happen:
A bad earth connection between Mobo and chassis
Memory timings/voltages
Graphics Card fault or Graphics card not seated properly or possibly another add-on card causing trouble.
Faulty HDD (s)
When you get everything back and you put it together again make sure that the Mobo is seated properly in the case and that there is nothing between the stand-offs and the board. Dont put any add-on cards in (except the graphics card) and use only one stick of RAM. Don't install the DVD/CD drive or hard-drive at this stage. See if the system boots. If it doesn't swap the RAM stick with another one and try again.
If it does and you can get into bios, re-set bios to optimized defaults.
Then add other parts one at a time (I know this is a long and boring process) and reboot after each component is fitted until you find something that makes the system loop.