The 2006 RRS had an air compressor with a weak intake valve spring. The spring wouldn't keep the valve closed and allowed air to escape which caused the compressor to not produce enough pressure. Then the system would say "well, we didn't get the right pressure in the allotted time, let's go to slammed mode". I had this problem with mine and replaced the compressor with one from ebay for like $250. I think my replacement was off a 2008. Looks like you can get them on ebay starting at $170 now. Before going down that route, pull the suspension fault code and if it's the "pressure wasn't reached in time" fault. If it was, it's probably the compressor... or a big system leak somewhere. Replacing the compressor is totally doable your garage. If you have a Disco, you know how to work on LRs.
The bags seem to be pretty reliable AFAIK
. A leaky bag would make one corner low and the compressor would run all the time to keep it up. It'd show up as one corner sagging after it sits for a bit. You can get replacement bags for $430 on the front and $307 on the back each from Atlantic British. (They're on sale now) I'd replace the air shocks before converting to springs because the air ride is just so darn comfortable and capable in offroad height. It also has "Extended mode" for if it detects that it's high-centered on something. It'll push an extra few inches to try and get out of it. Really sweet system.
If it has the ACE system, inspect the sway bars and look for pop-can sized "thing" in the middle. This is the ACE "thing" that uses high pressure hydraulic fluid to resist cornering and keep the Range level. Works great. Check those can-looking things for leaks / weeps. Mine are starting to weep a bit and that's common for this age. If they are leaking badly, that can cause a suspension fault, but it'd show up as ACE in the dash. You don't want a Range with really leaky ACE swaybars. They're stupid-expensive. If they are weeping, keep an eye on them and keep the fluid topped up because the system can be trashed by running low.
Another good thing to know is that these Ranges lose their friggin mind if the battery is bad or if they sit for a long time without charging. Low voltage will cause a suspension fault because the computer doesn't get info from the height sensors. When this happens you usually see transmission, HDC, and suspension faults as well. Sometimes you see ACE faults too, but that seems to only happen around 10v.
My RRS is a monster offroad. It's got triple locking diffs and amazing traction control. It's an absolute blast to own!