Unfortunately, it looks like plugging your card into the 4th PCIe slot puts it in x8 mode. However, the real difference in speed between x8 and x16 is negligible, so you should be fine regardless.

Your reboots may be unrelated, but are you getting bluescreens or something to help debug the issue? Make sure your BIOS and your drivers are up-to-date.
Also, random reboots may be OC-related, so if you're overclocking much, maybe you could try toning it down a bit.

Thank you for your answers. The slot PCIe x16/x8_3 is indeed that I'm using. I don't know if it is working x8 or x16, but based on benchmarks results, this slot does not slow down my GPU.

I have random reboots and I'm trying to narrow down the problem. I don't think it is a drivers problem, as sometimes the PC reboots from Windows and then it cannot go past BIOS - it just go into a reboot loop. Sometimes it reboots after several hours or even one day. I also tried almost everything in BIOS settings (default settings, OC turned on and off, etc.). Of course I have the latest BIOS version taken from this forum. I changed the whole RAM from Corsair to Kingston, changed the PSU and so on. The only things that I didn't change were: CPU and GPU. In the weekend I finally changed the GPU to my old one and since then - no reboots. I'm still waiting - maybe the restarts will finally happen, but if not, I will be wondering what is wrong. This can of course be a GPU, but I'm using this GPU in my old PC now and no reboots so far. Maybe the GPU wasn't fit tight enough in the PCIE slot? Or the PSU is malfunctioning, although it is a Corsair AX860i, so it should be fine. I suppose that this is a mainboard that I should RMA, but I still don't have enough proof for that.