My 1990 240 sedan w/215k automatic seems to coast in drive. Driving on the level freeway at 65mph, let off the gas, and the engine slows to idle while the car is "coasting" at high speed in overdrive. Step on the gas again and when the engine rpm,s catch up to your speed, the car accelerates. There are no other drivability problems. The transmission works fine in all other respects. The trans fluid was just changed and is full to the d.s. mark. It behaved the same before the change and after. Has anyone had this "phenomenon"? Is it a sign of something bad coming? Thanks in advance.