I purchased a 91 945 Turbo wagon this weekend and on the trip home (about 300 miles)after about 200 miles of trouble-free cruising at 70-75 mph the car started to lose power and I noticed a lot of black smoke out the exhaust. I pulled over to check under the hood, left the engine idling, but could find nothing obvious that might be the cause. While sitting there, the car idled fine. I got back in and drove the rest of the way home with no more problems. Looking through the service records that came with the car I found this has been a recurring, intermittent problem for the last few years, but by the time the previous owner got the car to the shop it was running fine. Any ideas on where to start looking for a cause?