I have a 1991 300D with 386,000 miles. The engine is strong, however it won't shift into high gear. It pulls reverse fine. In drive it starts fine, and shifts from 1st thur a few grears then when it is ready to shift at 40-45mph and 3,000rpm on shift. I have changed the filter and fuild but no change. Any ideas?? With almost 400,000 miles not sure how much it will cost and if it is worth it? It is a great car I hate say good bye but 45mph is slow going. Thanks, Jim