When the automatic transmission on my 740 wagon shifts into the highest gear, the rpm's don't drop back down to customary levels but instead remain in what feels like the stratosphere: at 55mph, about 3,700 rpms; at 45 mph, about 2,800 rpms, etc. The engine sounds as though I'm flying a WWI bomber when this occurs. This happened a few times about a month ago and again a week ago. 85% of the mileage I put on this machine is stop and go city driving but when I do take it on the highway, this is a disturbing phenomenon. What's happening with the transmission? How is this diagnosed and remedied? Thanks in advance for any ideas/suggestions/clues.