power steering on a late model

[phat46]

My '02 dodge Ram truck has been doing some weird things lately with the power steering. After it sits for a few hours it doesn't have any assist. Once you drive a few feet the power assist comes back. I checked the fluid level after it did it today when i left work and the fluid was only very slightly low, but I did notice when i opened the resevior it made a sound like it was under pressure. The trucks under warrentee so it's not a big deal,  I was just curious as to what might make it do this.