yes I have this problem as well. its between 60-75 mph that is fluctuates. I've brought it to the dealer once and they said they couldnt duplicate the problem. I also brought it in to fix my sunroof from rattling while its openned to vent. they said the same thing ("cant duplicate issue") WTF!! did you even TAKE it for a test drive?
I tried tightening the sunroof, but its tight and still rattles.
I know a few other people with the same issue with their speedo.
I noticed on my car on a recent road trip, the trip odo was out when compared to one of those "test" sections on the highway. It was out less than .1 km/km but it was out noticably by the end of the test section.
Stock car 2 months old with 4500km on the car, no burnouts to affect tire wear...