1989 420 Sel Mystery

I'M posting this just to see what you guys think about it. I was talking to my sister who owns a very nice looking 89' 420 SEL and asked her about her car since she was not driving it and told me that is tired of it, here is the problem: driving around the city on the streets, runs fine but any time she gets on the freeway and keeps speeds over 55 mph, after maybe 20 or 30 minutes the car dies and She has to coast for the nearest shoulder in the freeway then let it rest for about 30 minutes, then it'll start again and the story repeats again, so now she drives only on the streets only, and then told me that already took it to a couple of german car specialists? and after about 1,800 dlls. still does the same thing, any ideas? or a good mechanic in the Anaheim Ca. area where to take it? thanks!!. I say; is this a mystery?.