Salut,
Just search for "Reset" in the forum and you will find a lot of posts for the same problem. It is quite common.
Normally it is related to a weak 5V supply for the CPU. If this power falls below a critic level, the machine resets to prevent any damage.
There are several possible reason for this.
Bad connectors, bad bridge rectifiers, capacitors or voltage regulators on the PDB, etc.
You will find all you need in the forum.