BlueOx1

I am having a problem with my 2002 excursion 7.3 4x4 .The steering while driving down the road will move left off and on every 2-3 seconds for maybe 1/2 mile to as long as 5 miles ,won't do it every time it's driven but alot of times.I have flushed fluid in the power steering system , I have changed the gear boxes thinking that was the culprit( got a redhead steering box) ,I am considering changing the pump next because I have not find anything that helps with this problem .Any suggestions would help I don't want to just keep throwing parts at it.I have looked at my ball joints and tie rods and they all seem tight . So like I said any suggestions would help.