The gearbox is designed to operate like that.
What you were experiencing was the torque converter lock-up clutch engaging and dissengaging. When the load on the box is light, the clutch is engaged and the revs are at it's lowest. When the load increases to beyond a threashold value, the clutch dissengages and the revs jump up by about 200-300rpm.
I compare it to when I'm riding a bicycle. When I sit and easily spin the pedals, is the condition under which the clutch will be locked up. When the going gets tuff and I need to exert more force, I get up out of the seat to pedal harder - that is when the clutch will dissengage.
The cruise control doesn't foresee a hill, it reacts when the speed starts to drop. It instructs the engine to deliver more power to quicly get back to the preset speed. That is when the clutch will dissengage untill the set speed is reached.
We have trained our rightfoot to accelerate slighty on the way into the hill and we don't mind a drop in speed of maybe 5 or 10 km/h to conserve fuel and to keep the box comfy.
What I do when I tow my Echo Chobe (which weighs the same as a Gothic Cathedral - with earodynamics by a brick factory) is to liberally employ 4th gear. 4th Gear gives about 3000rpm at 110km/h. I have tested and found the "stall speed" for my clutch to be at 3000rpm - in other words above 3000rpm the clutch doesn't dissengage anymore. This means I can happily ask the engine and gearbox to do the job and that 3.8V6 MIVEC
motor is a pleasure at 3000rpm. I only grab 5th when the road is level and will remain so for a good few kms.In other words I try to look after my gearbox by asking the motor to work a little bit harder. It doesn't seem to make any difference to the fuel consumption - either way it melts the petrol card
.