New 420R SV, 2600 miles. Was on the final 20 miles of a 100 mile run and on empty dual carriageway making (cough) good progress. Proceeded to accelerate from the 12.30pm position on the speedo through to the 2pm position, which was met with vibrations and lack of shove as if the car could no longer push itself through the air. Tried a couple of times and got the same. It was as if the fuel was down to fumes or it was on 2 cylinders. Came off slip road in 3rd, acceleration fine as it was when joining next dual carriageway. Up into 4th car accelerated through to 2pm position on speedo no problem. Slowed down for cameras and was able to repeat the above again when back up to speed. Everything else fine - fuel, temp, oil pressure etc . Gremlin or something wrong? Thanks a lot in advance.