Thank you for your answer, Lsdmeasap.
Yes, I've flashed the backup properly and yesterday I proceeded with the procedure you described to clear the CMOS. This morning I've powered on the PC but, after a shut down, I've got the same issue. Then I've tried to go back to original BIOS, version F4, but this time I decided to flash it under DOS, with FlashSPI. Then, Ive loaded the optimized settings and I've updated the backup BIOS with ALT + F12. The problem occurred again, I do not know what other attempts I could do...
Unfortunately I have no other sticks of ram available for testing but the fact that the system, once started, works without problems for several hours leads me to think we can rule out a compatibility problem between the motherboard and memory modules. Instead, it seems that the motherboard is unable to provide the needed boost to start the system: like a car, which does not operate the starter...
I do not know if it can be a hardware defect or can be solved by changing some of the supply voltage within BIOS. I have not overclocked anything, also because the processor is a non-"k" and I am not familiar with motherboards based on P67 chipset. But I wonder if there might be a voltage equivalent of the Core2Duo Vdroop?!?