The difference between boxes could be a damaged down-lead from the LNB which could be getting internally wet when it rains rather than a reduction in signal received by the LNB. Does the degradation of the picture extend beyond the period when it is actually raining, possibly indication a damp cable problem rather than signal rain fade. With some cable it is possible for water to run down the hollow dielectric if the F connection at the LNB has been waterproofed.