Cisco Aironet Access Points

I have four Cisco Aironet 340 access points, and about 25 wireless devices that roam to each of these access points (warehouse/manufacturing faciltiy). The problem I am having is that each of the access points tend to “drop” associations at any given time, and which point the only way to re-associate any wireless devices with it is to reboot the AP. I have made a series of changes that have included, setting up each AP as a repeater in case it loses it’s backbone connectivity. I also changed the channel from 6 to 7. There are some 2.4Ghz two way radios I thought might be interferring. Has anyone seen this problem or suggest anything else? Thanks.