Hello Claudio,
How many APs do you have running Unleashed 200.3 code? If 3 or more, your network shouldchoose which 2.4 and 5GHz channels to use. Because 2.4GHz goes thru walls and around otherthings better, your devices (who make the decision) may be choosing the strongest signal, and ithappens it on your 2.4GHz band.
If you reboot the AP they prefer, or turn it off temporarily, which other AP(s) do your clients choose,and can you see any difference in throughput? This is what I'm thinking...