1994 Lincoln Continental - Rear air shocks lower when car is parked for a while and each night.

The compressor comes on each time the key is turned on and pumps up the two rear air shocks to the correct height. It takes about 2 minutes each time. The car rides fine then except the compressor comes on and off more often than it should to keep the rear shocks at the correct height. The front shocks are always at the correct height. What should I do? Thanks.