Dodgy Disco air suspension

I have a 2003 discovery TD5 auto diesel. Problem with air suspension. It goes down on both sides when stopped (but not all the time). Always re-inflates after running engine for a few minutes and then next time you stop it may go down or it may not ! Sometimes it stays up for a day or two and other times will go down as soon as you stop the car. Had understood that if air bags had gone then it would go down every time car stopped, but if valve problem then would not go up when started. Any suggestions ? Thanks