Searching more on the internet, I've found that this bug seems to be happening on several different distros, so probably is a firmware problem. Still don't know how to solve it, but got these two messages on dmesg:
iwlagn 0000:0e:00.0: Microcode SW error detected. Restarting 0x82000000.
iwlagn 0000:0e:00.0: iwl_tx_agg_start on ra
Usually after these messages I cannot initiate new TCP/ICMP connections or the internet becomes really slow. I've put, after reading this forum from Red Hathttps://bugzilla.redhat.com/show_bug.cgi?id=519154#c11
the file /etc/modprobe.d/iwlagn.conf, with
options iwlagn swcrypto=1
and will test to see if it helps the problem.