2004 Cadillac Deville Power Steering Failu8res

Since new the power steering intermittently locks up when driven through water, oftentimes even small puddles. After the fourth time at the dealers
the steering box was supposedly replaced. Belt is checked frequently. Problem has continued/ My 72 year old wife is the driver of the vehicle and when the problem has occured she has almost lost total control before getting the vehicle stopped. Dealer says they can noty find anything wrong.