At your mileage, it would be a good idea to replace your nitrogen cells, which are a component of your self-leveling suspension. They are often the cause of a problem like yours, and even if they're not completely shot they're due to be replaced.
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car