Suspension problem
2000 Mercedes Benz S500 V8 Two Wheel Drive Automatic 180000 miles

I have S500 that for the last 3 month off and on has a message light that the arimatic are too low to stop the car the message is red. as i said it comes off and on. If I have to lift the car with a jack, when I bring the car down the car would not come up right away and it never comes up completely. Do I have a problem with the Shock or is it the Air compressor that does not have enought pressure to charge the shock. How can I tell before I spend all the money on one or the other? Any advice would be greatly appreciated since parts on this cars are so expensives.....

thank you
Mike.