Don't rule the compressor out as of yet as the source of the problem. I was having similar problems with mine. It would not rise and then it would. Erratic ride height behavior. Or, IRHB. Well just made that one up but I found that the compressor was working but it was very week and it took a...

Ouch. I was presented with this option on my 02 but opted out as I did the castration job on my 99 Disco and was disappointed with the ride. Never again for me. I think it is worth the effort to salvage the system if you have the bucks.