had the exact same problem on an old peugot here, where it would have power steering then lose it, then it would come back,
turned out to be a cracked hose which had pulled air into the system, the pump would hit air every so often and the stearing would go stiff until it cleared.
The older rangerovers are solid on the steering in cold weather until you have driven it for about 200 yards, (probably due to tired pump)
try replacing the fluid, and check carefuly for leaks.