JR rerceivers getting bad hits
There was a similar thread in the newsgroups about two years ago. It was with Futaba receivers, or at least it appeared that way (3 receivers involved). However, it was only on one channel, whcih they finally found has some interference. They never found the interference source, and just quit using that channel.
It has strong likings to this thread, in that other receivers were flying fine, and only two types of Futaba receivers were involved. Is it possible the variances in designs of the different manufacturers receivers leave some types of interference or signals more prone to causing errors only in certain conditions and on certain designs? Maybe the other receivers are just skating by on a narrow margin, and if the source of the interference changes, they may all start getting hit. The pond seems to be a factor as well, and that could contribute to the whole mess.
What JR receiver types were involved? In the other situation, it was limited to two versions of Futabas receivers if I remember correctly (a google search should show the threads, and you might be able to glean some info from it).
It could just be one of those things. Unusual circumstances, everything comes together in just the right alignment, and one design is just a tad more susceptible than others. The fact that there are thousands and thousands of JR receivers out there with limited problems means it would probably take a small crew of people with the right electronics to find out why this is happening.
The most important thing to do is find the source of interference. That is where the problem is, and it sounds as if you are on top of it already.