Hi Stefano,
That kind of behavior would lead me to think it is a bad solder joint somewhere because of the rig having to "warm up". The question would be where. You might could isolate it to a area using a plastic or non-metallic tool by pressing slightly in various ares of the boards. Visual inspection may help as well, but there are mounted parts on both sides and removing a board would be necessary to examine both sides. SMD connections will develop the same type of cold joints that through hole parts will.
73,
Mitch
N4MF
