How much rev-hunting are we talking about? 500, 2000, more? Does it free-rev as if it's in neutral then 'bite'?
Without you having highlighted any other noticeable issues, I'd agree with post #2 from quinnj3 in that it's simply searching for a gear due to deciding which it needs to be in for optimum power. Try the same hill/speed in manual mode with it put into 6th and see if it does the same. If the revs drop too low and the car can't make it in 6th, it will automatically drop down to 5th, even in manual mode. If the revs are then at ~2000, the car would usually change back into 6th if in auto mode. (Does that make sense?). 5th and 6th are very close until you get to or above around 120kph, therefore that gear change is very smooth. Also, try it with the cruise control doing the driving, as that will hold a constant speed, ruling out slight deviations in speed that might cause a 'false' gear change, as you want the revs to be doing the gear changes, not the speed.
A flared gear (been there, done that) is very violent. It essentially doesn't engage the next gear, and continues revving hard (assuming you're on the throttle) until it suddenly finds the gear, at which point all the built up power (usually lots of it) is dumped into the gearbox. It feels like someone hitting the back of your car and the engine trying to jump out of the bonnet at the same time.
All that being said, if paragraph two doesn't work out, I'd still get Fiat/MultiECUScan and check for any codes stored in the system. Clear them, go out and repeat your 100kph hill run and see if they reappear.
Another potential possibility. I don't have a DPF on mine, but would a regen cause this behaviour (the speed being enough), which is then being exacerbated by going uphill?