6 cylinder outback over heating while towing.

I recently towed a U haul small box trailer up in Nor Cal.  Every time I hit any length if hill the motor temps would slowly rise up, until I  had to pull over a number of times and open the hood and let it idle until the temps came back down.  I bought this Subaru Outback with the 6 cylinder engine just so I could tow small stuff, but  this cannot be normal.  As I was way below the max load limit.  All levels were fine during the whole trip.  Fans were running.  Why is it getting hot?  It is rated for more load than i had on it.  Everything runs fine on flat ground.  I checked the condenser and radiator and neither seem to be to badly plugged up.  i was running on 1st gear while going up, to ease the load.  Taking it easy and not pushing it.