bmw problem?

a few months ago the triangle with circle around light started coming on when doing about 60 -70 mph mainly when accelerating then when I eased off gas it went off now its jumping all time like the engines catching or touching breaks slightly specially going round roundabouts but no light comes on I took it to garage and man drove it but couldn't find anything wrong he did a diagnostic test and it came back with fault road speed signal but he didn know what this means. any ideas my car is a 318i 56 reg petrol.