The best conclusion that was collectively drawn was this was not perhaps the direct fault of the equipment, but a sensitive predisposition to water permeating any delamination of the radome structure ...
I wouldn't be surprised at that theory. Many years ago, a 737-200 I flew on South Pacific routes had on-going problems not picking up active CB outside 30 miles or so and even had trouble picking up large islands until within 50 miles. The techs in Hong Kong were unable to fault the radar Bendix RDR-1E.
During cruise at 35,000 in visual flight we bypassed a large storm easily but it did not appear on the radar screen. I took a photo of the CB and of the radar at same time and sent it to Bendix Co. in USA and asked for an opinion. They replied that very probably water was getting inside the radome and freezing at high altitudes but would unfreeze on descent into lower levels - around 12-15,000 in the Pacific.
Icing on the inside of the radome caused significant attenuation and thus reduced the effective range. In the warmer levels the ice melted and the radar would come good again with no fault found on the ground by the techs. The Bendix suggestion was that techs should check the seals and to bake the radome in a special oven until all the moisture was evaporated. After that, a check should be made for tiny holes in the radome that would let water in and the various seals around the radome.
That was done and the radar was perfect after that.