I have a 1998 Catera. Some things to check:
1) How does it feel when you go over a bump..is there a distinct "Clunk?" from the rear? If so, your rear shocks could be bad or are going bad/leaking.
2) The Air-Compressor for the suspension is starting to fail
3) Cracked, broken or leaking air lines coming FROM the compressor to the Rear shocks.
I know this because I replaced my rear shocks myself. It was really easy to do if you have mechanical abilities. Otherwise, you are looking about $200.00 labor and of course, those knuckleheads are going to try to gouge you for $600.00 for the shocks. Buy the Shocks at Schucks (Hee hee that's funny!) and then find a place that will install them for you. It is an easy job!
My leveling light continues to blink after a short distance, so I assume I have broken lines or a bad compressor. I will replace them all shortly.
Lastly, I was told just today that I "Might need to reprogram it" so that it does not continue to try to fix itself. I have to look into that too.
If you still have the original shocks, my bet is that you need new ones. DO NOT BE FOOLED BY THE OUTRAGEOUS PRICES for these. I got the Rear PAIR, that is right, BOTH OF THEM, for $80.00 at Schucks auto! It came with new T-fittings and lines. After I put them in, it is like I am riding on Air. If you do it yourself, you will need a friend to help you get the top-Shock nut off.
Note: I just replaced my front ones this past weekend too. My car is riding SMOOOOOOOTH now!