here in the last few weeks i have been having trouble with the power steering in my 99f250 2wheel drive when the rpms are at the bottom end - i have noticed that the rpms seem to be lower than what they used to be (idling around 600 rpm) - is this an rpm issue? (if so how do i adjust it)is it a power steering issue? (where do i start) is it a vaccuum issue? (where do i start)

any help is appreciated
thanks