Check AP positioning and ensure it's placed in a manner that gives
optimum coverage - remember and keep the AP's antenna at least 1
wavelength (12.5cm) away from reflective surfaces like metal.
I would then do a survey and find out the coverage from all of the 3rd party AP's.
You could try using Ekahaus free heatmapper tool:
http://www.ekahau.com/wifidesign/ekahau-heatmapperThen from the results I would look at the standards used, channel occupancy and signal levels in the areas of concern.
Note: If there are AP's using the lower rates i.e. 802.11b , then you may need an AP that uses AirtimeFairness to mitigate the probability of the DCF MAC issue - cue Ruckus AP's.If there aren't any 802.11b AP's, you may be able to just simply add in another Cisco/Linksys AP and shout louder than everyone else, making sure you try and use the least congested channels to help with airtime.