Thanks for the fast reply Alan! Yes, that was one of the first things I did - went from Channel Fly to Background Scanning with a scan time of about 3 hours. I tried a shorter timeframe first, but I believe it made it worse.
I can't say for sure until we know - the size and general description of the floors (mostly cubes, mostly offices, etc), - number of WLANs in the air - number of neighboring networks and the RSSI levels. With 75 users/floor, the floors are probably 10-15,000, maybe more. If so, you don't have enough APs and transmit power is probably too hot. I agree with Alan - never use Channel Fly. Other things to try: OFDM only BSS minrate to 12 Enable proxy arp Disable 802.11r Disable rogue detection (unless you really use monitor it and use it) Keep the number of WLANs as low as possible (2-3) Channel width 20 on 2.4 and auto on 5 Possibly consider enabling band steering