blinking power on Arduino Leonardo

Dear all,

First of all I’m new to the forum so I hope to be posting in the right section, if I’m not please tell me and I’ll move.
I have a technical question regarding arduino Hardware.

What I have is a +12v source that is converted to +5v by a stabilized LM2596 step down and I feed it to the +5v pin on the arduino in order to power it.

Now what I see is that if I connect the arduino to the stepdown the voltage on the +5v is oscillating between +2 and +5 at very low frequency (sub Hz, I can notice it using a multimeter), but the step down in itself does not seem to have any problem. Indeed if the arduino is not connected, but everything else is (I have a bunch of other cards connected to the sam step-down) everything seems to work fine. If, instead, I connect the arduino to the USB port everything works just fine, but I assume this is not a surprise. Of course without the USB power the arduino itself is powering on and off so it is not usable.

Has anyone ever had this issue before? do you know what might be broken?

thank you for your help!
F.