Disco 3 (LR3) Steering fault

My brother has a 2005 Disco 3 TDV6 with a strange fault on the steering. The problem is that the steering sometimes feels really light from cold and the vehicle seems to float around on the road, almost like my old Disco 1 before I replaced all of the front and rear suspension bushes. After two or three miles the feel returns to 'normal' but after say ten miles will intermittently tighten up for a couple of hundred yards then return to normal again.
The lower column universal joint has been changed in the last month and the problem persists. Rather than replace parts in a process of elimination (rack, pump, pipes etc.), does anyone have any experience of this problem please? It has been suggested that the pressure control valves in the rack could be faulty amongst other things.