My F350 has developed some issues that only show up when towing. After driving for about 30+ minutes, the wrench light comes on. We have no power loss, it seems to be fine, not straining or anything at all odd. Then if we are dealing with hills, after about the 4th or 5th grade, the temp will shoot straight up. We can pull over (or stop the incline) and it drops back down. This all happens in just a minute or two, at most.
No smoke, no loss of fluids.

Where should we start?

Thanks!