I have a cisco AIR-AP3802P-E-K9 AP at one of my client's offices, the AP sometimes unregisters from the controller although when I check the switch I see the AP still has an IP address but it no longer appears on the controller.
I have tried to shut nd unshut the port but if it registers, after a while it gets deregistered again.
The AP register to the WLC through a capwap tunnel which is build over an IP connection, so, physical problem can´t explain everything.
You can test the AP to WLC recheability with ping but probably you will not see the problem either. If you have some kind of monitoring tool, something that look at latency, drops,etc for a long period, you may see the problem.
One action you can take is change the AP tunnel sensibility by increase the valeu of " AP Retransmit Config Parameters"