2003 SL500 and 2 Mercedes dealerships cannot figure out the problem

I have a 2003 SL500 and 2 Mercedes dealerships cannot figure out the problem. The "Check Engine" light comes on and the car starts running poorly, no power, engine missing, bad exhaust smell. The first time this problem occured was in January 2008 and the car had approx. 18,000 miles; at least since the car is in my possession; I bought it pro-owned from a Mercedes dealership with 100,000 miles warranty. So, I thought I was safe. While driving on the freeway from the Seattle airport(late at night)the "check engine" light came on, the car lost power and I stopped and turned the engine off. I than restarted it and it drove very rough, had a strong exhaust smell and I had it towed by AAA to the Mercedes Dealership, Phil Smart, Seattle. The had the car in the shop from 1/21 until 2/13/08, did extensive testing and road test, did replace some items, but never confirmed what the actual problem was. I drove the car for approx. 5,000 miles and the exact same problem resurfaced while I was visiting in Canada. Now, I brought the car to Wilson Mercedes dealership, Bellingham, because they were closer. Again, the car drove rough and the service writer agreed that there is a problem. The next day they problem was gone and although they drove the car approx. 250 miles they cannot figure out the problem. It is still in the shop. Any idea????