I had a similar problem on an old discovery, I changed the pump with no effect, then the lower steering connection as I had a work UJ but that didnt help, I then changed the box and it was brand new. Mine also used to cut out and you had to sort of nudge the steering wheel before it picked up, similar feeling to a slipping belt. You should also check all the ball joints on the steering rods too specially the one at the bottom of the drop arm which is a critical one, if it goes you will loose all steering, have seen this happen and it really isnt funny.