Has anyone experienced level issues with a commercial instalation; where levels change at ram?

I encountered an issue at a restaurant: they use a DBX ZonePro 640 with a 2 channel Crown amplifier. The system has been in use for several years. Recently, levels started to go up/down at ramdom. I replaced wall mounted controls for ZonePro and it seemed to correct the issue for several weeks. a couple of weeks ago, volume started going up/down at ramdom. Has anyone encountered anything like this? There is no distorsion present, only the level changes from loud to low....