900 interference

We have a few 900mhz customers having problems dropping the connection. Most are LOS and high on the landscape. Where others on the same AP are having no problems. The ISP has tried adjusting the frequency with no results. Location is 100 Km East of Perth Western Australia.
Any Ideas what we could do to prevent the interference?
Please follow this link for details.

http://www.waldon.com.au/freq.htm