I seem to have the same problem always while slowing down/coasting and many times while comming down hill, quite scary... After an oil change it seemed to stop the problem but it once again has started up, its comming up for an oil change soon, would this have any effect on this problem? Also I do not understand what a high idle is as this is my first car :-\ Any help would be great thanks!

[Edit]
Forgot to mention I have a 1988 300e not a 1989 like the post states, but the symptoms are the same, the car stalls it becomes hard to steer off the road to restart the car and all of the lights on the HUD light up