Get it fixed asap. This *exact* problem occurred with a friend's Volvo and two hours after we were looking at it and scratching our heads the car caught fire and burned completely on the way home. He got out ok but had a pretty wild story to tell about flames erupting from under the hood (and all the way out the back) at 60mph. As nothing but a block and some twisted metal was left we never did figure out what was wrong but the posts above appear to make sense.
By the way - it was a 760GLe and was somewhere between 84-88...