Strange things by Wireless connection

I' m connected wireless with Internet server Distance ca 3 Km . The type of my antenne is Doradus Cb3 4040 2,4 GHZ 30x30.
If both stations send by channel 9 the connection is perfect
If we change to channel 6 there are problems ( very bad or no connection at all)
Unfortunately the connection has to take place on channel 6.
We have also tried other Hardware ( for example D-link but the problem is the same.)
Perhaps there is some jamming from an other Transmitter
Does anyone know what to do