Thanks for the Seasonic suggestions. The PSU is actually an AOpen one, with its own part number, but it's not available (or I'm not smart enough to find it...) in their web site. On the other hand their sales procedure seems to be quite complicated, so maybe trying to get a one through an easiest way could be the solution.
About CMOS battery, I already changed it. The frozen clock issue is now quite weird. When all this began, after the htpc crashed then clock and date ware set to a standard point in the past when rebooting. I then changed battery, and now the problem is different. When it crashes the clock time stops at the moment of the crash, but day, month, and year remain with the correct values in the present. So maybe, as you mention, the real problem is with the motherboard and not the PSU
... Then I thinking this HTPC is becoming the worst investment of my life...
I'll monitor with the software you recommended the PSU and the motherboard as well.
Thanks for helpful feedback, I really appreciate it