Thanks Greg (and Texas),
Come to think of it, this problem does seem to occur at lower revs. It is very slight, almost makes you think it is a bump (or bumps) in the road, but it isn't. I am due to see the mechanics for installation of an air filter, so I will ask them to do a diagnostic and I will let you know what they discover.
It's good to hear that this is apparently not an isolated incident. The mechanics will thus probably know what is going on.
Thanks again.
Anyone else have this experience?