Hi Springbok, I also used the standard supplied antenna outside and it only lasted a couple of months, when I took it apart, water/damp had penetrated the base, the copper wire hads corroded badly and disintegrated, chopping the doenlead into 6" sections, the damp penetration had gone right down to the SMA plug. Resultant effect on the RB was exactly what you're experiencing, so I would suggest if the first instance to dis-assemble the antenna base, peel off the sticky back plastic, and prise out the magnet/screen, and have a look!
I've since dis-assembled another 2 standard antennas, soldered them up properly, pumped them full of electricians' silicone grease (NOT silicone sealant, which'll eat copper) and one of them, has now been outside for some 8 months, and is providing superb signals.
I did write up a post on how to weather-proof the standard antenna, and will find the link if you want it, but before worrying about the Box have a good a look inside the antenna base.