You have had two good replies already but here s my $.02, At 9 PM or whenever this starts look at the combined nodes and find the culprit, verify this at the receiver go to the node and hunt like hell. Trans spike said it you may have a welder or a radio operator causing the problem it only happens when they do their thing, you will have to be out there when it happens try using leakage equip. it has helped me pinpoint problems in the past. Good luck