Jon
The section of the M5 between Weston-S-M and Bridgwater is just about the only place I regularly get weaving.
I suspect that this is down to sidewinds caused by the geography of the Severn estuary, Somerset levels, Mendip and Polden Hills.
I now always watch out for this on this section and I would guess this is the area you are talking about.
Look at the geography next time you are passing through.