That is weird, do the electric shower decrease the error rate while on every summer? also you say that electric fences cause increase error rate during summer, so I guess that for some reason the electric shower effect that happens doesn't appear during winter when the certain cause of higher error rate is not persistent (electric fences are not used), not sure if phone lines passing near electric fences and that cause electromagnetic interference on the lines (but in that case it is not reversible easily, may be the Zyxel "b10a"s models which include a line filter can help more) or the fences are affecting the house electricity itself which go through router adapter to inside the router causing interference or the router adapter is the cause of interference since it is probably near the router and increase error rates so something in the electric shower is canceling or decreasing that effect from house electricity while it is on (if all is on the same circuit).
Also do not forget that during summer phone lines affected by heat and that is a reason why errors may increase or even speed decreases especially on longer lines, this is aside from the your query.
It is guessing I am not an electrician may be it is all nonsense at all.