Sounds to me like the radome is the most probable culprit. Would be interesting to learn more about this. If someone here who has experienced this could send me some detailed info in a private message, the exact model number of aircraft, type of radar installed, altitudes, distance, location of occurences, I think I can get it to the right folks to look into the situation. Are you an ALPA carrier? If so, include that info as well. There may be a way to fix this.