I think it's down to that local thing Mr B, personally I don't think August is a fickle month up here (except on the rivers if there's no water) but I have seen it tough elsewhere, particularly the likes of the reservoirs down South.
I think I'd err on the side of saying it's a myth nationally but some truth that August doldrums exist at a local level (just like they do in every other month of the season
).
Now how's that for sitting on the fence
H