I have a 99 continental with the air leveling suspension. I had the same thing with the car sagging sometimes after being parked for a long time or over night. I think that is has to due with the bleeding system. Every time you shut it off it bleeds pressure out of the system and the valve either gets stuck open a little or it just bleeds out too much and then you have an embarrassing
looking car. I used to shut the system off in the trunk after it pumped up and it would stay level for a week then I would do it again. I asked the dealer if they could change the programming so it wouldn't bleed out any but they said that it couldn't be done. Now mine need replaced because they are old and cracked and seeping air out slowly.