Driving along the M4 just passed Swansea and overtook an S3. Only doing 80mph, so nothing too fast. Coming to the end of the M4 and noticed said S3 suddenly "bombing" up behind me as we approached the roundabout. I duly pulled into the inside lane to go around the roundabout. S3 decides he's going to go around the roundabout level with me. Once we get to the A48 (Dual Carriageway), S3 is off like a rocket. A couple of miles further on, I catch-up with the S3 (I'm doing 80mph), he must have slowed to 70mph. As I'm coming up behind him, he decides to pull out into the "fast" lane and sit there at 70mph. When he gets past the slower vehicle, he speeds up to 90mph as the road is clear. Then slows down to 70mph. Does this everytime I come close to him. Not sure why you S3 Owners may be a little touchy to us A3 Owners - thought we were all in the same Gang !!