Many thanks to all, especially to crew for their direct input. I shall feed back to my sister and her friend. I think the pax is early 70s and a very moderate drinker. It is a curious reaction she has and I now wonder if there was any kind of minor 'shimmy' in the fuselage.
As I recall, when the 340-600 first started the rear economy cabin was subject to a subtle movement that could hardly be detected - but upset a lot of pax 'middle ears'. I certainly recall in a 346 when I was in PE, looking out along the wing in the night and the craft was clearly 'wallowing'. The wing was rising and falling distinct against the background, although I coud not feel it as I was on the pivot point and the actual movement at my seat was very small.
But she is experiencing this on multiple carriers and craft.