Mine did that when the sensor took a crap. I have no idea how to check to see if its bad. This thread is a few years old... Might not be a bad idea to search around and see if there are certain voltage ranges for a sensor. Good luck!

 

I have a 99 expedition .. when i first got it only one side raised up and the pump ran for ever , one of the air bags had a leak so ... I bought a strut masters conversion kit for about $120 on ebay and had it installed for $80 (local mechanic) ... and now it runs great and really smooth .. glad i did it