Hi
You might want to upgrade.
There was similar problems with surface machines in particular, but it did affect other devices as well.
Have a look at this thread.
https://community.arubanetworks.com/t5/Wireless-Access/surface-device-disconnection-issues/td-p/305219/page/3
I had a customer with the same problem on surface devices, after upgrade the devices behaved as expected.
You should look into upgrading to 6.5.3.3 og 6.5.4.2
At the moment 6.5.4.8 is the latest on 6.5 and it's marked as conservative, so you should be pretty safe.
Roar