Mondovermona, thank you for your help. I wrote my latest post without reading tour answer so, yes, someone is still reading me :)
I made the tapping test you are suggesting, but after the issue appear. It could be a good idea to perform it before, in order to indue the issue. I will do that.
About the oscilloscope: yes, I have a 100MHz tektronics so I can definitely check if the cpu is running when the issue appear. The thing is I dont know which pin to check and what should I read... Any help on this would be much appreciated.