I have owned my GL 320 from new. On 3 occasions in the last month after travelling approximately 50 miles on the motorway at a variable speed of around 60 to 80mph the power suddenly reduces, it becomes a struggle to maintain 50mph. I have pulled off the motorway and the car over revs and moves forward gaining speed ver very slowly. Pulling over, turning the ignition off and re starting immediately cures the problem, until the next time. The vehicle has just been serviced, there are no warning lights illuninated and everything else appears normal. Any ideas?