interference on 900

Hello Im seeking a little help our wisp has been up for about 5 years now and we have used canopy 900(we are in a very hilly and dense tree area) since the beginning. We have really had no problem until the beginning of the year. We have 2 omni’s and three 120 panels. The 2 omni’s are on 2 different towers and the 120s are on another tower they are all about 5 miles apart and make a triangle. They are all on different feq except the panels which the ones that face away share the omni freq and they are all gps sync. This was done in Oct of last year. In Jan I had to change all the freq because one tower would not get good upload, So I moved them around and it worked till the end of Feb then one of the panels and one omni not on the same freq the panel is 906 and omni is 915 just started dropping and picking people back up and this would happen for 2 to 3 hrs then fine. After about 2 months it stopped then out of the blue started again. This time for 2 weeks and both omni’s and 2 panels. Then gone. Now three weeks later both omni’s. Only for a hr or two. If i change the omni on 915 to 906 or 923 its good for the day then the next day i have to change it again. I drove around with a spectrum anly and found something scanning 906 to 920 I went back the next day and it was gone. We work with the city so I know its not the scada. By where I saw the hoping I found a helicopter office that has a Motorola cdm 1550 ls but they say its on vhf. Any ideas??