In my experience with air suspension it's rarely a fault with the compressor, unless there's an air leak causing the compressor to run too much causing it to burn out. The compressors do run randomly, as you say, sometimes on start up but other times not. If you're sure you don't have an airbag leaking I would re-visit the sensors. I recently got rid off my air setup and replaced it with coils, best decision I've made with this car.