So all Sevilles from the 90's have active suspension? But from '00 and onward, only the STS does?
I think the deal is the rear of the car, is self-leveling whether you have sport [active] suspension or not (SLS or STS).
So if something fails in terms of a sensor or compressor, the car feels the need to limit you in case it's off balance or something (rear too low or too high).
Do a search on accessing the onboard diagnostics, and then post back the codes you get from the suspension system. Someone may know the codes and let you know where it's being reported from. It could be a bad connection or something more serious.
But worry-not... it is not a CVRSS strut or anything so expensive since you don't have CVRSS (continuously variable road sensing suspension) aka Sport suspension.
So to summarize... you have "passive struts" and a self-leveling rear-end (adjusts for height with varying payloads).
if you had an STS.... you would have "Active" struts and the self leveling rear-end.