Below are some high level initial troubleshooting steps:
1) Segregate if the problem is seen in wired network itself or only in WLAN.
2) Check if the issue is band specific - 2.4 or 5 GHz
3) Have a separate VLAN for WLAN to reduce broadcast/ multicast pollute the RF.
4) Ensure you dont have non wifi RF interference.
5) You are in a really old code. Go through the release notes and atleast move to the latest 5.0.4.x code.