How to debug problems on startup, debian 9

I feel a little lost here.

Not sure it's the right community to ask, anyway:

I have installed Debian 9 in a new office that I have assembled. When I restart, 4/5 times, it remains stuck on a black screen, past in front. Other times, it starts perfectly.

As it hangs, I have to perform a hard reset until it works, so that I can not read the bad logs to dmesg.

I thought it was perhaps a graphics driver problem, so I installed AMD proprietary drivers without success.

Equipment:

ASROCK 990fx extreme4
AMD FX 8350
AMD GeForce GTX 260
SSD 240gb