OK, I just picked up a 1988 420 SEL from a family friend that passed away. I got the car for $1000.00. I decided to take it on the expressway to see how it rode. It rode beautifully, except for one thing. While cruising along at 60 mph, the tranny decides to downshift on its own. Obviously thats not good, are there sensors in the transmission that went bad? Or would I need a complete rebuild?