Certainly worth getting the P-codes read if you can get it to a dealer. The GM 5L40-E transmission fitted in your Range Rover has something of a reputation for torque converter lock-up clutch failures at higher mileages. The clutch lining breaks up and contaminates the transmission fluid which then clogs up the spools in the valve block preventing the clutches from engaging properly. The ECU detects the fault from the speed sensors and (at best) drops you into limp-home mode. It might then clear itself for a while to return again later.
I suggest you drain the fluid and examine it for contamination. A fluid & filter change might buy you a little more time but, if it is the torque converter at fault, it will have to be replaced/remanufactured.
What mileage has your Range Rover racked up? The consensus on here seems to be that you can expect this type of failure to crop up at around 80,000-100,000 miles.
Phil