I had a very similar problem which worsened over time. If I let the vehicle sit for a week it would be sitting right on the bump stops. The problem was the pressure tank that the compressor pumps into. It had rusted on the top where you can't see it because it holds snow and road salt in the winter. This was only diagnosed after the compressor and all ride height sensors were replaced. The tank itself only cost $300 to replace but it cost me over $1500 to get there.