Wlan intermittent packet loss

[jokuji]

I have setup a network as below.

(  pfsensebox  )
WAN –---------  intel nic 1

intel nic 2  LAN  -------- a switch ------------ lan client1,2,3........

intel nic 3 OPT1 ------- a cisco router nat disabled as a AP  ---------  wireless client1,2,3.....

The issue is that wireless users are intermittently experiencing packet loss and high latency to the wlan NIC ip. The cisco AP router is working has no issues, the NICs are all intels. So not sure what is causing this. This is proving to be quite annoying as the lan users have no issues at all. Anyone has faced this issue before ? Any idea how to troubleshoot this ?

Thanks