Hi shaik,
Troubleshooting wireless issues like this can be a bit involved to find the root cause.
To answer your first question, I did some poking around in the diag commands and so far I do not see a way to pull the CPU utilization directly from the FortiGate CLI.
In general CPU utilization on AP' s it not an issue, however if you want to check it, you can use http/telnet to the IP address of the AP. Default login is admin and no password, this can be changed of course.
The GUI will probably be far easier to spot check. If you do see high utilization, please open a case with TAC so that we can troubleshoot appropriately.
Regardless of that, please ensure you are on the latest patch for your major version of fortiOS. For FOS 5.0 this is currently FAP 5.0.7, and for FOS 5.2.0 it is currently FAP 5.2.0
If you still experience issues please open a TAC case so that we can debug this and get you fixed up.
Cheers!
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Sean Toomey, CISSP FCNSP
Consulting Security Engineer (CSE)
FORTINET— High Performance Network Security