intermittent Suspension sinking, help required

Hi,
I'm posting this on behalf of my friend who owns a 2006 Disco 3 TDV6
His suspension seems to have an intermittent fault causing the air springs to exhaust all the air.
He has visited his local indy who fitted a new compressor £475 thank you.
Also had all software updated and no faults showing on diagnostics.
Now here is the intermittent fault.
Car stays up for 10 hours no problems then suddenly for no reason the systems seems to wake up and exhaust all the air so the cars arse is on the floor??
Start car up and it raises straight away & drives with no faults. switch off and still sits proud then intermittently out the blue it's back on its bump stops.
Any ideas folks:crazy:

If it's anything like a P38, a leaking airspring, height sensor or corroded connector.
If it's like an L322, one of the valve block assemblies. they have made the new ones so complicated that it will be like looking for a needle in a haystack.

as long as it doesnt trigger any fault code and being so internmittent the most obvious is a leaking bag cos there used to self-seal from time to time....but as said it could be a valve block too(cos it's more similar with L322 than P38)...it must be diagnosed by someone who knows what he's doing... the guys who fitted a new compressor for intermittent dropping rear are complete schmucks, IMO for these kind of problems you can get more help there http://www.disco3.co.uk/forum/