Power steering getting hard
 

I noticed my power steering is getting difficult to turn, pump is full, fluid looks clean, no leaks anywhere. It worked fine nice and easy for the last few years,
ever since it got below 0 outside, its been acting up. I keep it my heated shop,thinking its acting up because of the cold. Its in the hi 40's now.
If I raise the rpm's up it gets easier, the belts are tight.
Does this sound like a pump or steering box problem?
It's a '86 K-5 w/ a replacement 350, instead of the 305. Its pretty much stock.

replace the power steering pump first (its the cheapest) sounds like its not making enough pressure. the steering boxes are usually pretty tough and dont have many problems.