The air system on the LS, or any luxury car, is so complex that diagnosing faults is very difficult, unless it's an obvious component failure like a leaking shock absorber. Also, one part failing is very likely to lead to another due to age. Having had three LS430's I know the most common suspension problems by far are strut leaks or height sensor issues. If I was in that situation again I wouldn't hesitate in converting to conventional springs, that way your suspension woes are mostly behind you. Most LS430's sold in the US were fitted with springs and people still loved the ride quality, just like the LS400.