Its a long shot, vac advance not working?
I'm thinking along the same lines of the problem being associated with vacuum advance plus centrifugal advance. Under the conditions where you are running along the dual carriageway at say 60 mph there will be 10 - 14 degrees of vacuum advance plus 17 -22 degrees of centrifugal advance. It's round about the maximum you can get. Under these conditions the L.T, wire to the points ( and also the baseplate earthing flexi; lead ) will be at maximum stretch, so I'd be checking these for breakage etc. ( see photo above ). There again I would have expected that giving the car some welly would reduce the vacuum and the problem would go away. And the rev; counter would be jumping around which it isn't.
I've got a decent strobe gun so will check out the advance and vacuum next time I can get under the bonnet. As Tony Dollyman said, it might be a long wait. Meanwhile the garage is occupied by my Z3 which has a broken front spring. Grrr. And the spring punctured the new tyre. Double Grrrr.