There are bursts at hourly intervals throughout the day, and the only variation I've seen is a later start on Saturday morning - apart from that it seems to be persistent.
I haven't changed equipment, to give rise to this new pattern, and can so far spot nothing to account for it.
I'm not noticing problems with the internet link apart from this - browsing and so on seem unaffected.
The pics linked to are from routerstats reading from my main Netgear DG834G.
I'm not sure what to try.