2004 Chevy Venture steering tension feels eratic

I just bought a used chevy venture. Cars runs great. However in driving I sometimes notice that the steering goes from a stiff feeling to a loose feeling then back to stiff. Especially when making curves and small adjustments in direction. Is this normal for the Chevy venuter? If not I was thinking that this could be caused by a slipping tensionner belt or erratic power steering pump. Car has sat in lots for almost 9 moths and I have just purchased car and serviced basic fluids.