I removed the CPU and checked the pins. Actually, one of them was slightly bent (the one on the upper left corner of the inner square on the socket) but I'm not sure if the bending happened when I removed the CPU or before because the corresponding mark on the CPU was exactly in the center. Some other marks were a bit off, but every pin was marked in some ways. I took some pictures with my phone but unfortunately they were not very clear, since there is no a macro setting.
I pushed the pin slightly with a stick and it returned in line with the others. I refitted the CPU and tested my system again. Same results. Perfectly working with C1E and C3/C6/C7 disabled. With C1E enabled, system starts, seems stuttering and choppy, after a few seconds either an application crashes or a pop up window comes up saying there is some data corruption on disk, or I experience a BSOD.
I may be wrong but I have the feeling that the problem wasn't there, at least it wasn't obvious and visible. I received a new beta BIOS from Gigabyte Technical support, I will try with that. If it doesn't work, well, maybe I really have a faulty motherboard. Or could it be a bad CPU?
Thanks for your help. Please let me know if you have other suggestions or ideas on the issue.