I have a few solutions for you, most of which can be utilized together, but some require hardware and software solutions.
Identify your biggest WLAN & other interferers
-walk around with a spectrum analyzer or Wireless tool
-identify the source
-use a directional to locate the offenders, for use with below solutions
-identify any other devices that may be operating in the same frequency ranges, or similar ranges, such as microwaves, specific wireless phones (not cells), lights, Zigbee, wireless video cameras, bluetooth, or other.
-identify which channels they operate on
Increasing the power of your WAPs
-add an amplifier
-use a WAP with more power output
Increasing the antenna gain of your WAPs
-add a high db antenna
-use an antenna with a footprint tailored to your clients
Increasing the power of your clients
-use higher power WLAN NICs
-alter the transmission power of your WLAN NICs via settings
Increasing the antenna gain of your clients
-use WLAN NICs with higher gain
-use external antennas
-use tailored antennas tuned to the type (omni, yagi, etc)
Blocking interfering RF / Replace interfering devices
-use RF blocking paint
-use RF blocking wallpaper (copper sheath, etc - google "faraday cage wallpaper" for samples
-identify other faraday cage options (there are several).
-if you identified any devices operating around 2.4 in the identification step, consider replacing them, or blocking them at this point.
Identify vendors specializing in RF saturated environments
- Some vendors like Cisco's Cleanair product line, and others sell products meant specifically for high RF environments like your own. You may be able to do a WAP and client swap in order to help with the situation. They will also offer loaner equipment for you to test out in your environment.
Better placement of your WAPs
-use some wireless auditing tools to determine strength
-use some tools (vendors provide for free or use google) to come up with alternative locations
-test alternative placements
There are companies that a wireless survey including much of above for around $3,000 to $5,000 US but I would save the cash and do it myself.