Dangerous power steering problem

I have a 1996 Gnd Caravan. I dont necessarily think this is a dodge problem but hee it goes.
The power steering periodically goes out while driving. Not like you are low on fluid or stiff. Like an old steering box goes bad, I have to turn ~180 degrees to get action, Then things seem ok stiff but controllable.
Now my wheel is 180 out from normal. These episodes are becomming more frequent. Is this a rack problem or a pump problem, I would appericate some help on my next investment...
Thanks in advance